Image encoding/decoding method and apparatus, and recording medium for storing bitstream

ABSTRACT

Disclosed herein is an image encoding/decoding method and apparatus. The image decoding method according to the present invention includes obtaining symmetric motion vector difference mode availability information from a bitstream, obtaining first prediction direction&#39;s zero motion vector difference information from the bitstream, obtaining symmetric motion vector difference mode information of a current block from the bitstream on the basis of the symmetric motion vector difference mode availability information and the first prediction direction&#39;s zero motion vector difference information, obtaining first prediction direction&#39;s reference picture index information, second prediction direction&#39;s reference picture index information and a first prediction direction&#39;s motion vector difference value on the basis of the symmetric motion vector difference mode information, and generating a prediction block of the current block by using at least one of the first prediction direction&#39;s reference picture index information, the second prediction direction&#39;s reference picture index information and the first prediction direction&#39;s motion vector difference value.

TECHNICAL FIELD

The present invention relates to an image encoding/decoding method andapparatus, and a recording medium for storing a bitstream.

BACKGROUND ART

Recently, the demand for high resolution and quality images such as highdefinition (HD) or ultra-high definition (UHD) images has increased invarious applications. As the resolution and quality of images areimproved, the amount of data correspondingly increases. This is one ofthe causes of increase in transmission cost and storage cost whentransmitting image data through existing transmission media such aswired or wireless broadband channels or when storing image data. Inorder to solve such problems with high resolution and quality imagedata, a high efficiency image encoding/decoding technique is required.

There are various video compression techniques such as an interprediction technique of predicting the values of pixels within a currentpicture from the values of pixels within a preceding picture or asubsequent picture, an intra prediction technique of predicting thevalues of pixels within a region of a current picture from the values ofpixels within another region of the current picture, a transform andquantization technique of compressing the energy of a residual signal,and an entropy coding technique of allocating frequently occurring pixelvalues with shorter codes and less occurring pixel values with longercodes.

DISCLOSURE Technical Problem

The present invention may provide a method and apparatus for deriving anMVD by considering an enhanced performance condition, when a symmetricMVD deriving method is used to improve an encoding/decoding efficiency.

An object of the present invention is to provide a method and apparatusfor encoding and decoding a picture, which improve an encoding anddecoding efficiency.

Another object of the present invention is to provide a recording mediumstoring a bitstream generated by an image decoding method or apparatusaccording to the present invention.

Technical Solution

An image decoding method according to an embodiment of the presentinvention comprises obtaining symmetric motion vector difference modeavailability information from a bitstream; obtaining first predictiondirection's zero motion vector difference information from thebitstream; obtaining symmetric motion vector difference mode informationof a current block from the bitstream on the basis of the symmetricmotion vector difference mode availability information and the firstprediction direction's zero motion vector difference information;obtaining first prediction direction's reference picture indexinformation, second prediction direction's reference picture indexinformation and a first prediction direction's motion vector differencevalue on the basis of the symmetric motion vector difference modeinformation; and generating a prediction block of the current block byusing at least one of the first prediction direction's reference pictureindex information, the second prediction direction's reference pictureindex information and the first prediction direction's motion vectordifference value, wherein the obtaining of the first predictiondirection's reference picture index information, the second predictiondirection's reference picture index information and the first predictiondirection's motion vector difference value obtains the first predictiondirection's reference picture index information, the second predictiondirection's reference picture index information and the first predictiondirection's motion vector difference value by deriving rather thandecoding from the bitstream when the symmetric motion vector differencemode information of the current block indicates a symmetric motionvector difference mode.

In the image decoding method of the present invention, wherein the firstprediction direction is L1 prediction direction, and the secondprediction direction is L0 prediction direction.

In the image decoding method of the present invention, wherein the firstprediction direction's zero motion vector difference informationindicates that the motion vector difference value of the firstprediction direction is not decoded but derived as (0, 0).

In the image decoding method of the present invention, wherein the firstprediction direction's zero motion vector difference information isobtained at a picture level.

In the image decoding method of the present invention, wherein thesymmetric motion vector difference mode availability information isobtained at a sequence level.

In the image decoding method of the present invention, wherein, when thesymmetric motion vector difference mode information of the current blockindicates a symmetric motion vector difference mode, the firstprediction direction's motion vector difference value is derived basedon a second prediction direction's motion vector difference value of thecurrent block.

In the image decoding method of the present invention, wherein, when thesymmetric motion vector difference mode information of the current blockindicates a symmetric motion vector difference mode, the firstprediction direction's reference picture index information is derived asan index of a backward reference picture that is nearest to a currentpicture in a first prediction direction's reference picture list, andthe second prediction direction's reference picture index information isderived as an index of a forward reference picture that is nearest tothe current picture in a second prediction direction's reference picturelist.

In the image decoding method of the present invention, wherein, when thesymmetric motion vector difference mode information of the current blockindicates a symmetric motion vector difference mode, the firstprediction direction's reference picture index information is derived asan index of a forward reference picture that is nearest to a currentpicture in a first prediction direction's reference picture list, andthe second prediction direction's reference picture index information isderived as an index of a backward reference picture that is nearest tothe current picture in a second prediction direction's reference picturelist.

In the image decoding method of the present invention, wherein, when thesymmetric motion vector difference mode information of the current blockindicates a symmetric motion vector difference mode, the firstprediction direction's reference picture index information and thesecond prediction direction's reference picture index information arederived as an index of a short-term reference picture.

An image encoding method according to an embodiment of the presentinvention comprises determining symmetric motion vector difference modeavailability information; determining first prediction direction's zeromotion vector difference information; encoding symmetric motion vectordifference mode information of a current block on the basis of thesymmetric motion vector difference mode availability information and thefirst prediction direction's zero motion vector difference information;and determining, on the basis of the symmetric motion vector differencemode information, whether or not first prediction direction's referencepicture index information, second prediction direction's referencepicture index information and a first prediction direction's motionvector difference value are encoded, wherein the determining of whetheror not the first prediction direction's reference picture indexinformation, the second prediction direction's reference picture indexinformation and the first prediction direction's motion vectordifference value are encoded is determined as not encode the firstprediction direction's reference picture index information, the secondprediction direction's reference picture index information and the firstprediction direction's motion vector difference value, when thesymmetric motion vector difference mode information of the current blockindicates a symmetric motion vector difference mode.

In the image encoding method of the present invention, wherein the firstprediction direction is L1 prediction direction, and the secondprediction direction is L0 prediction direction.

In the image encoding method of the present invention, wherein the firstprediction direction's zero motion vector difference informationindicates that the motion vector difference value of the firstprediction direction is not encoded but derived as (0, 0).

In the image encoding method of the present invention, wherein the firstprediction direction's zero motion vector difference information isencoded at a picture level.

In the image encoding method of the present invention, wherein thesymmetric motion vector difference mode availability information isencoded at a sequence level.

In the image encoding method of the present invention, wherein, when thesymmetric motion vector difference mode information of the current blockindicates a symmetric motion vector difference mode, the firstprediction direction's motion vector difference value is derived basedon a second prediction direction's motion vector difference value of thecurrent block.

In the image encoding method of the present invention, wherein, when thesymmetric motion vector difference mode information of the current blockindicates a symmetric motion vector difference mode, the firstprediction direction's reference picture index information is derived asan index of a backward reference picture that is nearest to a currentpicture in a first prediction direction's reference picture list, andthe second prediction direction's reference picture index information isderived as an index of a forward reference picture that is nearest tothe current picture in a second prediction direction's reference picturelist.

In the image encoding method of the present invention, wherein, when thesymmetric motion vector difference mode information of the current blockindicates a symmetric motion vector difference mode, the firstprediction direction's reference picture index information is derived asan index of a forward reference picture that is nearest to a currentpicture in a first prediction direction's reference picture list, andthe second prediction direction's reference picture index information isderived as an index of a backward reference picture that is nearest tothe current picture in a second prediction direction's reference picturelist.

In the image encoding method of the present invention, wherein, when thesymmetric motion vector difference mode information of the current blockindicates a symmetric motion vector difference mode, the firstprediction direction's reference picture index information and thesecond prediction direction's reference picture index information arederived as an index of a short-term reference picture.

A non-transitory computer readable recording medium storing a bitstreamgenerated by an image encoding method according to an embodiment of thepresent invention, wherein the image encoding method comprisesdetermining symmetric motion vector difference mode availabilityinformation; determining first prediction direction's zero motion vectordifference information; encoding symmetric motion vector difference modeinformation of a current block on the basis of the symmetric motionvector difference mode availability information and the first predictiondirection's zero motion vector difference information; and determining,on the basis of the symmetric motion vector difference mode information,whether or not first prediction direction's reference picture indexinformation, second prediction direction's reference picture indexinformation and a first prediction direction's motion vector differencevalue are encoded, wherein the determining of whether or not the firstprediction direction's reference picture index information, the second 1o prediction direction's reference picture index information and thefirst prediction direction's motion vector difference value are encodedis determined as not encode the first prediction direction's referencepicture index information, the second prediction direction's referencepicture index information and the first prediction direction's motionvector difference value, when the symmetric motion vector differencemode information of the current block indicates a symmetric motionvector difference mode.

Advantageous Effects

According to the present invention, it is possible to provide an imageencoding/decoding method and apparatus with improved compressionefficiency. The present invention may improve an image encoding/decodingefficiency by providing an MVD deriving method and apparatus consideringan enhanced performance condition, when a symmetric MVD deriving methodis used.

According to the present invention, an image encoding/decoding methodand apparatus with enhanced encoding and decoding efficiency may beprovided.

In addition, according to the present invention, a recording mediumstoring a bitstream generated by an image encoding method or apparatusof the present invention may be provided.

In addition, according to the present invention, a recording mediumstoring a bitstream, which is received and decoded by a picture decodingapparatus of the present invention and is used to reconstruct a picture,may be provided.

DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram showing a configuration of an encodingapparatus according to an embodiment to which the present invention isapplied.

FIG. 2 is a block diagram showing a configuration of a decodingapparatus according to an embodiment and to which the present inventionis applied.

FIG. 3 is a view schematically showing a partition structure of an imagewhen encoding and decoding the image.

FIG. 4 is a view showing an intra-prediction process.

FIG. 5 is a diagram illustrating an embodiment of an inter-pictureprediction process.

FIG. 6 is a diagram illustrating a transform and quantization process.

FIG. 7 is a diagram illustrating reference samples capable of being usedfor intra prediction.

FIG. 8 is a flowchart illustrating an image encoding method according toan embodiment of the present invention.

FIG. 9 is a flowchart illustrating an image decoding method according toan embodiment of the present invention.

FIG. 10 is a view illustrating spatial neighbor blocks of anencoding/decoding target block.

FIG. 11 is a view illustrating temporal neighbor blocks of anencoding/decoding target block.

FIG. 12 is a view for explaining the history-based motion vectorcandidate derivation.

FIG. 13 to FIG. 18 are views for explaining various embodiments ofsyntax and semantics related to a symmetric MVD mode according to thepresent invention.

FIG. 19 is a flowchart for explaining an image decoding method accordingto an embodiment of the present invention.

FIG. 20 is a view for explaining an image encoding method according toan embodiment of the present invention.

MODE FOR INVENTION

A variety of modifications may be made to the present invention andthere are various embodiments of the present invention, examples ofwhich will now be provided with reference to drawings and described indetail. However, the present invention is not limited thereto, althoughthe exemplary embodiments can be construed as including allmodifications, equivalents, or substitutes in a technical concept and atechnical scope of the present invention. The similar reference numeralsrefer to the same or similar functions in various aspects. In thedrawings, the shapes and dimensions of elements may be exaggerated forclarity. In the following detailed description of the present invention,references are made to the accompanying drawings that show, by way ofillustration, specific embodiments in which the invention may bepracticed. These embodiments are described in sufficient detail toenable those skilled in the art to implement the present disclosure. Itshould be understood that various embodiments of the present disclosure,although different, are not necessarily mutually exclusive. For example,specific features, structures, and characteristics described herein, inconnection with one embodiment, may be implemented within otherembodiments without departing from the spirit and scope of the presentdisclosure. In addition, it should be understood that the location orarrangement of individual elements within each disclosed embodiment maybe modified without departing from the spirit and scope of the presentdisclosure. The following detailed description is, therefore, not to betaken in a limiting sense, and the scope of the present disclosure isdefined only by the appended claims, appropriately interpreted, alongwith the full range of equivalents to what the claims claim.

Terms used in the specification, ‘first’, ‘second’, etc. can be used todescribe various components, but the components are not to be construedas being limited to the terms. The terms are only used to differentiateone component from other components. For example, the ‘first’ componentmay be named the ‘second’ component without departing from the scope ofthe present invention, and the ‘second’ component may also be similarlynamed the ‘first’ component. The term ‘and/or’ includes a combination ofa plurality of items or any one of a plurality of terms.

It will be understood that when an element is simply referred to asbeing ‘connected to’ or ‘coupled to’ another element without being‘directly connected to’ or ‘directly coupled to’ another element in thepresent description, it may be ‘directly connected to’ or ‘directlycoupled to’ another element or be connected to or coupled to anotherelement, having the other element intervening therebetween. In contrast,it should be understood that when an element is referred to as being“directly coupled” or “directly connected” to another element, there areno intervening elements present.

Furthermore, constitutional parts shown in the embodiments of thepresent invention are independently shown so as to representcharacteristic functions different from each other. Thus, it does notmean that each constitutional part is constituted in a constitutionalunit of separated hardware or software. In other words, eachconstitutional part includes each of enumerated constitutional parts forconvenience. Thus, at least two constitutional parts of eachconstitutional part may be combined to form one constitutional part orone constitutional part may be divided into a plurality ofconstitutional parts to perform each function. The embodiment where eachconstitutional part is combined and the embodiment where oneconstitutional part is divided are also included in the scope of thepresent invention, if not departing from the essence of the presentinvention.

The terms used in the present specification are merely used to describeparticular embodiments, and are not intended to limit the presentinvention. An expression used in the singular encompasses the expressionof the plural, unless it has a clearly different meaning in the context.In the present specification, it is to be understood that terms such as“including”, “having”, etc. are intended to indicate the existence ofthe features, numbers, steps, actions, elements, parts, or combinationsthereof disclosed in the specification, and are not intended to precludethe possibility that one or more other features, numbers, steps,actions, elements, parts, or combinations thereof may exist or may beadded. In other words, when a specific element is referred to as being“included”, elements other than the corresponding element are notexcluded, but additional elements may be included in embodiments of thepresent invention or the scope of the present invention.

In addition, some of constituents may not be indispensable constituentsperforming essential functions of the present invention but be selectiveconstituents improving only performance thereof. The present inventionmay be implemented by including only the indispensable constitutionalparts for implementing the essence of the present invention except theconstituents used in improving performance. The structure including onlythe indispensable constituents except the selective constituents used inimproving only performance is also included in the scope of the presentinvention.

Hereinafter, embodiments of the present invention will be described indetail with reference to the accompanying drawings. In describingexemplary embodiments of the present invention, well-known functions orconstructions will not be described in detail since they mayunnecessarily obscure the understanding of the present invention. Thesame constituent elements in the drawings are denoted by the samereference numerals, and a repeated description of the same elements willbe omitted.

Hereinafter, an image may mean a picture configuring a video, or maymean the video itself. For example, “encoding or decoding or both of animage” may mean “encoding or decoding or both of a moving picture”, andmay mean “encoding or decoding or both of one image among images of amoving picture.”

Hereinafter, terms “moving picture” and “video” may be used as the samemeaning and be replaced with each other.

Hereinafter, a target image may be an encoding target image which is atarget of encoding and/or a decoding target image which is a target ofdecoding. Also, a target image may be an input image inputted to anencoding apparatus, and an input image inputted to a decoding apparatus.Here, a target image may have the same meaning with the current image.

Hereinafter, terms “image”, “picture, “frame” and “screen” may be usedas the same meaning and be replaced with each other.

Hereinafter, a target block may be an encoding target block which is atarget of encoding and/or a decoding target block which is a target ofdecoding. Also, a target block may be the current block which is atarget of current encoding and/or decoding. For example, terms “targetblock” and “current block” may be used as the same meaning and bereplaced with each other.

Hereinafter, terms “block” and “unit” may be used as the same meaningand be replaced with each other. Or a “block” may represent a specificunit.

Hereinafter, terms “region” and “segment” may be replaced with eachother.

Hereinafter, a specific signal may be a signal representing a specificblock. For example, an original signal may be a signal representing atarget block. A prediction signal may be a signal representing aprediction block. A residual signal may be a signal representing aresidual block.

In embodiments, each of specific information, data, flag, index, elementand attribute, etc. may have a value. A value of information, data,flag, index, element and attribute equal to “0” may represent a logicalfalse or the first predefined value. In other words, a value “0”, afalse, a logical false and the first predefined value may be replacedwith each other. A value of information, data, flag, index, element andattribute equal to “1” may represent a logical true or the secondpredefined value. In other words, a value “1”, a true, a logical trueand the second predefined value may be replaced with each other.

When a variable i or j is used for representing a column, a row or anindex, a value of i may be an integer equal to or greater than 0, orequal to or greater than 1. That is, the column, the row, the index,etc. may be counted from 0 or may be counted from 1.

Description of Terms

Encoder: means an apparatus performing encoding. That is, means anencoding apparatus.

Decoder: means an apparatus performing decoding. That is, means andecoding apparatus.

Block: is an M×N array of a sample. Herein, M and N may mean positiveintegers, and the block may mean a sample array of a two-dimensionalform. The block may refer to a unit. A current block my mean an encodingtarget block that becomes a target when encoding, or a decoding targetblock that becomes a target when decoding. In addition, the currentblock may be at least one of an encode block, a prediction block, aresidual block, and a transform block.

Sample: is a basic unit constituting a block. It may be expressed as avalue from 0 to 2^(Bd)−1 according to a bit depth (B_(d)). In thepresent invention, the sample may be used as a meaning of a pixel. Thatis, a sample, a pel, a pixel may have the same meaning with each other.

Unit: may refer to an encoding and decoding unit. When encoding anddecoding an image, the unit may be a region generated by partitioning asingle image. In addition, the unit may mean a subdivided unit is when asingle image is partitioned into subdivided units during encoding ordecoding. That is, an image may be partitioned into a plurality ofunits. When encoding and decoding an image, a predetermined process foreach unit may be performed. A single unit may be partitioned intosub-units that have sizes smaller than the size of the unit. Dependingon functions, the unit may mean a block, a macroblock, a coding treeunit, a code tree block, a coding unit, a coding block), a predictionunit, a prediction block, a residual unit), a residual block, atransform unit, a transform block, etc. In addition, in order todistinguish a unit from a block, the unit may include a luma componentblock, a chroma component block associated with the luma componentblock, and a syntax element of each color component block. The unit mayhave various sizes and forms, and particularly, the form of the unit maybe a two-dimensional geometrical figure such as a square shape, arectangular shape, a trapezoid shape, a triangular shape, a pentagonalshape, etc. In addition, unit information may include at least one of aunit type indicating the coding unit, the prediction unit, the transformunit, etc., and a unit size, a unit depth, a sequence of encoding anddecoding of a unit, etc.

Coding Tree Unit: is configured with a single coding tree block of aluma component Y, and two coding tree blocks related to chromacomponents Cb and Cr. In addition, it may mean that including the blocksand a syntax element of each block. Each coding tree unit may bepartitioned by using at least one of a quad-tree partitioning method, abinary-tree partitioning method and ternary-tree partitioning method toconfigure a lower unit such as coding unit, prediction unit, transformunit, etc. It may be used as a term for designating a sample block thatbecomes a process unit when encoding/decoding an image as an inputimage. Here, the quad-tree may mean a quarternary-tree.

When the size of the coding block is within a predetermined range, thedivision is possible using only quad-tree partitioning. Here, thepredetermined range may be defined as at least one of a maximum size anda minimum size of a coding block in which the division is possible usingonly quad-tree partitioning. Information indicating a maximum/minimumsize of a coding block in which quad-tree partitioning is allowed may besignaled through a bitstream, and the information may be signaled in atleast one unit of a sequence, a picture parameter, a tile group, or aslice (segment). Alternatively, the maximum/minimum size of the codingblock may be a fixed size predetermined in the coder/decoder. Forexample, when the size of the coding block corresponds to 256×256 to64×64, the division is possible only using quad-tree partitioning.Alternatively, when the size of the coding block is larger than the sizeof the maximum conversion block, the division is possible only usingquad-tree partitioning. Herein, the block to be divided may be at leastone of a coding block and a transform block. In this case, informationindicating the division of the coded block (for example, split_flag) maybe a flag indicating whether or not to perform the quad-treepartitioning. When the size of the coding block falls within apredetermined range, the division is possible only using binary tree orternary tree partitioning. In this case, the above description of thequad-tree partitioning may be applied to binary tree partitioning orternary tree partitioning in the same manner.

Coding Tree Block: may be used as a term for designating any one of a Ycoding tree block, Cb coding tree block, and Cr coding tree block.

Neighbor Block: may mean a block adjacent to a current block. The blockadjacent to the current block may mean a block that comes into contactwith a boundary of the current block, or a block positioned within apredetermined distance from the current block. The neighbor block maymean a block adjacent to a vertex of the current block. Herein, theblock adjacent to the vertex of the current block may mean a blockvertically adjacent to a neighbor block that is horizontally adjacent tothe current block, or a block horizontally adjacent to a neighbor blockthat is vertically adjacent to the current block.

Reconstructed Neighbor block: may mean a neighbor block adjacent to acurrent block and which has been already spatially/temporally encoded ordecoded. Herein, the reconstructed neighbor block may mean areconstructed neighbor unit. A reconstructed spatial neighbor block maybe a block within a current picture and which has been alreadyreconstructed through encoding or decoding or both. A reconstructedtemporal neighbor block is a block at a corresponding position as thecurrent block of the current picture within a reference image, or aneighbor block thereof.

Unit Depth: may mean a partitioned degree of a unit. In a treestructure, the highest node (Root Node) may correspond to the first unitwhich is not partitioned. Also, the highest node may have the leastdepth value. In this case, the highest node may have a depth of level 0.A node having a depth of level 1 may represent a unit generated bypartitioning once the first unit. A node having a depth of level 2 mayrepresent a unit generated by partitioning twice the first unit. A nodehaving a depth of level n may represent a unit generated by partitioningn-times the first unit. A Leaf Node may be the lowest node and a nodewhich cannot be partitioned further. A depth of a Leaf Node may be themaximum level. For example, a predefined value of the maximum level maybe 3. A depth of a root node may be the lowest and a depth of a leafnode may be the deepest. In addition, when a unit is expressed as a treestructure, a level in which a unit is present may mean a unit depth.

Bitstream may mean a bitstream including encoding image information.

Parameter Set: corresponds to header information among a configurationwithin a bitstream. At least one of a video parameter set, a sequenceparameter set, a picture parameter set, and an adaptation parameter setmay be included in a parameter set. In addition, a parameter set mayinclude a slice header, a tile group header, and tile headerinformation. The term “tile group” means a group of tiles and has thesame meaning as a slice.

An adaptation parameter set may mean a parameter set that can be sharedby being referred to in different pictures, subpictures, slices, tilegroups, tiles, or bricks. In addition, information in an adaptationparameter set may be used by referring to different adaptation parametersets for a subpicture, a slice, a tile group, a tile, or a brick insidea picture.

In addition, regarding the adaptation parameter set, differentadaptation parameter sets may be referred to by using identifiers ofdifferent adaptation parameter sets for a subpicture, a slice, a tilegroup, a tile, or a brick inside a picture.

In addition, regarding the adaptation parameter set, differentadaptation parameter sets may be referred to by using identifiers ofdifferent adaptation parameter sets for a slice, a tile group, a tile,or a brick inside a subpicture.

In addition, regarding the adaptation parameter set, differentadaptation parameter sets may be referred to by using identifiers ofdifferent adaptation parameter sets for a tile or a brick inside aslice.

In addition, regarding the adaptation parameter set, differentadaptation parameter sets may be referred to by using identifiers ofdifferent adaptation parameter sets for a brick inside a tile.

Information on an adaptation parameter set identifier may be included ina parameter set or a header of the subpicture, and an adaptationparameter set corresponding to the adaptation parameter set identifiermay be used for the subpicture.

The information on the adaptation parameter set identifier may beincluded in a parameter set or a header of the tile, and an adaptationparameter set corresponding to the adaptation parameter set identifiermay be used for the tile.

The information on the adaptation parameter set identifier may beincluded in a header of the brick, and an adaptation parameter setcorresponding to the adaptation parameter set identifier may be used forthe brick.

The picture may be partitioned into one or more tile rows and one ormore tile columns.

The subpicture may be partitioned into one or more tile rows and one ormore tile columns within a picture. The subpicture may be a regionhaving the form of a rectangle/square within a picture and may includeone or more CTUs. In addition, at least one or more tiles/bricks/slicesmay be included within one subpicture.

The tile may be a region having the form of a rectangle/square within apicture and may include one or more CTUs. In addition, the tile may bepartitioned into one or more bricks.

The brick may mean one or more CTU rows within a tile. The tile may bepartitioned into one or more bricks, and each brick may have at leastone or more CTU rows. A tile that is not partitioned into two or moremay mean a brick.

The slice may include one or more tiles within a picture and may includeone or more bricks within a tile.

Parsing: may mean determination of a value of a syntax element byperforming entropy decoding, or may mean the entropy decoding itself.

Symbol: may mean at least one of a syntax element, a coding parameter,and a transform coefficient value of an encoding/decoding target unit.In addition, the symbol may mean an entropy encoding target or anentropy decoding result.

Prediction Mode: may be information indicating a mode encoded/decodedwith intra prediction or a mode encoded/decoded with inter prediction.

Prediction Unit: may mean a basic unit when performing prediction suchas inter-prediction, intra-prediction, inter-compensation,intra-compensation, and motion compensation. A single prediction unitmay be partitioned into a plurality of partitions having a smaller size,or may be partitioned into a plurality of lower prediction units. Aplurality of partitions may be a basic unit in performing prediction orcompensation. A partition which is generated by dividing a predictionunit may also be a prediction unit.

Prediction Unit Partition: may mean a form obtained by partitioning aprediction unit.

Reference picture list may refer to a list including one or morereference pictures used for inter prediction or motion compensation.There are several types of usable reference picture lists, including LC(List combined), L0 (List 0), L1 (List 1), L2 (List 2), L3 (List 3).

Inter prediction indicator may refer to a direction of inter prediction(unidirectional prediction, bidirectional prediction, etc.) of a currentblock. Alternatively, it may refer to the number of reference picturesused to generate a prediction block of a current block. Alternatively,it may refer to the number of prediction blocks used at the time ofperforming inter prediction or motion compensation on a current block.

Prediction list utilization flag indicates whether a prediction block isgenerated using at least one reference picture in a specific referencepicture list. An inter prediction indicator can be derived using aprediction list utilization flag, and conversely, a prediction listutilization flag can be derived using an inter prediction indicator. Forexample, when the prediction list utilization flag has a first value ofzero (0), it means that a reference picture in a reference picture listis not used to generate a prediction block. On the other hand, when theprediction list utilization flag has a second value of one (1), it meansthat a reference picture list is used to generate a prediction block.

Reference picture index may refer to an index indicating a specificreference picture in a reference picture list.

Reference picture may mean a reference picture which is referred to by aspecific block for the purposes of inter prediction or motioncompensation of the specific block. Alternatively, the reference picturemay be a picture including a reference block referred to by a currentblock for inter prediction or motion compensation. Hereinafter, theterms “reference picture” and “reference image” have the same meaningand can be interchangeably.

Motion vector may be a two-dimensional vector used for inter predictionor motion compensation. The motion vector may mean an offset between anencoding/decoding target block and a reference block. For example, (mvX,mvY) may represent a motion vector. Here, mvX may represent a horizontalcomponent and mvY may represent a vertical component.

Search range may be a two-dimensional region which is searched toretrieve a motion vector during inter prediction. For example, the sizeof the search range may be M×N. Here, M and N are both integers.

Motion vector candidate may refer to a prediction candidate block or amotion vector of the prediction candidate block when predicting a motionvector. In addition, a motion vector candidate may be included in amotion vector candidate list.

Motion vector candidate list may mean a list composed of one or moremotion vector candidates.

Motion vector candidate index may mean an indicator indicating a motionvector candidate in a motion vector candidate list. Alternatively, itmay be an index of a motion vector predictor.

Motion information may mean information including at least one of theitems including a motion vector, a reference picture index, an interprediction indicator, a prediction list utilization flag, referencepicture list information, a reference picture, a motion vectorcandidate, a motion vector candidate index, a merge candidate, and amerge index.

Merge candidate list may mean a list composed of one or more mergecandidates.

Merge candidate may mean a spatial merge candidate, a temporal mergecandidate, a combined merge candidate, a combined bi-predictive mergecandidate, or a zero merge candidate. The merge candidate may includemotion information such as an inter prediction indicator, a referencepicture index for each list, a motion vector, a prediction listutilization flag, and an inter prediction indicator.

Merge index may mean an indicator indicating a merge candidate in amerge candidate list. Alternatively, the merge index may indicate ablock from which a merge candidate has been derived, among reconstructedblocks spatially/temporally adjacent to a current block. Alternatively,the merge index may indicate at least one piece of motion information ofa merge candidate.

Transform Unit: may mean a basic unit when performing encoding/decodingsuch as transform, inverse-transform, quantization, dequantization,transform coefficient encoding/decoding of a residual signal. A singletransform unit may be partitioned into a plurality of lower-leveltransform units having a smaller size. Here,transformation/inverse-transformation may comprise at least one amongthe first transformation/the first inverse-transformation and the secondtransformation/the second inverse-transformation.

Scaling: may mean a process of multiplying a quantized level by afactor. A transform coefficient may be generated by scaling a quantizedlevel. The scaling also may be referred to as dequantization.

Quantization Parameter: may mean a value used when generating aquantized level using a transform coefficient during quantization. Thequantization parameter also may mean a value used when generating atransform coefficient by scaling a quantized level duringdequantization. The quantization parameter may be a value mapped on aquantization step size.

Delta Quantization Parameter: may mean a difference value between apredicted quantization parameter and a quantization parameter of anencoding/decoding target unit.

Scan: may mean a method of sequencing coefficients within a unit, ablock or a matrix. For example, changing a two-dimensional matrix ofcoefficients into a one-dimensional matrix may be referred to asscanning, and changing a one-dimensional matrix of coefficients into atwo-dimensional matrix may be referred to as scanning or inversescanning.

Transform Coefficient: may mean a coefficient value generated aftertransform is performed in an encoder. It may mean a coefficient valuegenerated after at least one of entropy decoding and dequantization isperformed in a decoder. A quantized level obtained by quantizing atransform coefficient or a residual signal, or a quantized transformcoefficient level also may fall within the meaning of the transformcoefficient.

Quantized Level: may mean a value generated by quantizing a transformcoefficient or a residual signal in an encoder. Alternatively, thequantized level may mean a value that is a dequantization target toundergo dequantization in a decoder. Similarly, a quantized transformcoefficient level that is a result of transform and quantization alsomay fall within the meaning of the quantized level.

Non-zero Transform Coefficient: may mean a transform coefficient havinga value other than zero, or a transform coefficient level or a quantizedlevel having a value other than zero.

Quantization Matrix: may mean a matrix used in a quantization process ora dequantization process performed to improve subjective or objectiveimage quality. The quantization matrix also may be referred to as ascaling list.

Quantization Matrix Coefficient: may mean each element within aquantization matrix. The quantization matrix coefficient also may bereferred to as a matrix coefficient.

Default Matrix: may mean a predetermined quantization matrixpreliminarily defined in an encoder or a decoder.

Non-default Matrix: may mean a quantization matrix that is notpreliminarily defined in an encoder or a decoder but is signaled by auser.

Statistic Value: a statistic value for at least one among a variable, acoding parameter, a constant value, etc. which have a computablespecific value may be one or more among an average value, a sum value, aweighted average value, a weighted sum value, the minimum value, themaximum value, the most frequent value, a median value, an interpolatedvalue of the corresponding specific values.

FIG. 1 is a block diagram showing a configuration of an encodingapparatus according to an embodiment to which the present invention isapplied.

An encoding apparatus 100 may be an encoder, a video encoding apparatus,or an image encoding apparatus. A video may include at least one image.The encoding apparatus 100 may sequentially encode at least one image.

Referring to FIG. 1, the encoding apparatus 100 may include a motionprediction unit 111, a motion compensation unit 112, an intra-predictionunit 120, a switch 115, a subtractor 125, a transform unit 130, aquantization unit 140, an entropy encoding unit 150, a dequantizationunit 160, an inverse-transform unit 170, an adder 175, a filter unit180, and a reference picture buffer 190.

The encoding apparatus 100 may perform encoding of an input image byusing an intra mode or an inter mode or both. In addition, encodingapparatus 100 may generate a bitstream including encoded informationthrough encoding the input image, and output the generated bitstream.The generated bitstream may be stored in a computer readable recordingmedium, or may be streamed through a wired/wireless transmission medium.When an intra mode is used as a prediction mode, the switch 115 may beswitched to an intra. Alternatively, when an inter mode is used as aprediction mode, the switch 115 may be switched to an inter mode.Herein, the intra mode may mean an intra-prediction mode, and the intermode may mean an inter-prediction mode. The encoding apparatus 100 maygenerate a prediction block for an input block of the input image. Inaddition, the encoding apparatus 100 may encode a residual block using aresidual of the input block and the prediction block after theprediction block being generated. The input image may be called as acurrent image that is a current encoding target. The input block may becalled as a current block that is current encoding target, or as anencoding target block.

When a prediction mode is an intra mode, the intra-prediction unit 120may use a sample of a block that has been already encoded/decoded and isadjacent to a current block as a reference sample. The intra-predictionunit 120 may perform spatial prediction for the current block by using areference sample, or generate prediction samples of an input block byperforming spatial prediction. Herein, the intra prediction may meanintra-prediction,

When a prediction mode is an inter mode, the motion prediction unit 11 lmay retrieve a region that best matches with an input block from areference image when performing motion prediction, and deduce a motionvector by using the retrieved region. In this case, a search region maybe used as the region. The reference image may be stored in thereference picture buffer 190. Here, when encoding/decoding for thereference image is performed, it may be stored in the reference picturebuffer 190.

The motion compensation unit 112 may generate a prediction block byperforming motion compensation for the current block using a motionvector. Herein, inter-prediction may mean inter-prediction or motioncompensation.

When the value of the motion vector is not an integer, the motionprediction unit 111 and the motion compensation unit 112 may generatethe prediction block by applying an interpolation filter to a partialregion of the reference picture. In order to perform inter-pictureprediction or motion compensation on a coding unit, it may be determinedthat which mode among a skip mode, a merge mode, an advanced motionvector prediction (AMVP) mode, and a current picture referring mode isused for motion prediction and motion compensation of a prediction unitincluded in the corresponding coding unit. Then, inter-pictureprediction or motion compensation may be differently performed dependingon the determined mode.

The subtractor 125 may generate a residual block by using a differenceof an input block and a prediction block. The residual block may becalled as a residual signal. The residual signal may mean a differencebetween an original signal and a prediction signal. In addition, theresidual signal may be a signal generated by transforming or quantizing,or transforming and quantizing a difference between the original signaland the prediction signal. The residual block may be a residual signalof a block unit.

The transform unit 130 may generate a transform coefficient byperforming transform of a residual block, and output the generatedtransform coefficient. Herein, the transform coefficient may be acoefficient value generated by performing transform of the residualblock. When a transform skip mode is applied, the transform unit 130 mayskip transform of the residual block.

A quantized level may be generated by applying quantization to thetransform coefficient or to the residual signal. Hereinafter, thequantized level may be also called as a transform coefficient inembodiments.

The quantization unit 140 may generate a quantized level by quantizingthe transform coefficient or the residual signal according to aparameter, and output the generated quantized level. Herein, thequantization unit 140 may quantize the transform coefficient by using aquantization matrix.

The entropy encoding unit 150 may generate a bitstream by performingentropy encoding according to a probability distribution on valuescalculated by the quantization unit 140 or on coding parameter valuescalculated when performing encoding, and output the generated bitstream.The entropy encoding unit 150 may perform entropy encoding of sampleinformation of an image and information for decoding an image. Forexample, the information for decoding the image may include a syntaxelement.

When entropy encoding is applied, symbols are represented so that asmaller number of bits are assigned to a symbol having a high chance ofbeing generated and a larger number of bits are assigned to a symbolhaving a low chance of being generated, and thus, the size of bit streamfor symbols to be encoded may be decreased. The entropy encoding unit150 may use an encoding method for entropy encoding such as exponentialGolomb, context-adaptive variable length coding (CAVLC),context-adaptive binary arithmetic coding (CABAC), etc. For example, theentropy encoding unit 150 may perform entropy encoding by using avariable length coding/code (VLC) table. In addition, the entropyencoding unit 150 may deduce a binarization method of a target symboland a probability model of a target symbol/bin, and perform arithmeticcoding by using the deduced binarization method, and a context model.

In order to encode a transform coefficient level (quantized level), theentropy encoding unit 150 may change a two-dimensional block formcoefficient into a one-dimensional vector form by using a transformcoefficient scanning method.

A coding parameter may include information (flag, index, etc.) such assyntax element that is encoded in an encoder and signaled to a decoder,and information derived when performing encoding or decoding. The codingparameter may mean information required when encoding or decoding animage. For example, at least one value or a combination form of aunit/block size, a unit/block depth, unit/block partition information,unit/block shape, unit/block partition structure, whether to partitionof a quad-tree form, whether to partition of a binary-tree form, apartition direction of a binary-tree form (horizontal direction orvertical direction), a partition form of a binary-tree form (symmetricpartition or asymmetric partition), whether or not a current coding unitis partitioned by ternary tree partitioning, direction (horizontal orvertical direction) of the ternary tree partitioning, type (symmetric orasymmetric type) of the ternary tree partitioning, whether a currentcoding unit is partitioned by multi-type tree partitioning, direction(horizontal or vertical direction) of the multi-type three partitioning,type (symmetric or asymmetric type) of the multi-type tree partitioning,and a tree (binary tree or ternary tree) structure of the multi-typetree partitioning, a prediction mode (intra prediction or interprediction), a luma intra-prediction mode/direction, a chromaintra-prediction mode/direction, intra partition information, interpartition information, a coding block partition flag, a prediction blockpartition flag, a transform block partition flag, a reference samplefiltering method, a reference sample filter tab, a reference samplefilter coefficient, a prediction block filtering method, a predictionblock filter tap, a prediction block filter coefficient, a predictionblock boundary filtering method, a prediction block boundary filter tab,a prediction block boundary filter coefficient, an intra-predictionmode, an inter-prediction mode, motion information, a motion vector, amotion vector difference, a reference picture index, a inter-predictionangle, an inter-prediction indicator, a prediction list utilizationflag, a reference picture list, a reference picture, a motion vectorpredictor index, a motion vector predictor candidate, a motion vectorcandidate list, whether to use a merge mode, a merge index, a mergecandidate, a merge candidate list, whether to use a skip mode, aninterpolation filter type, an interpolation filter tab, an interpolationfilter coefficient, a motion vector size, a presentation accuracy of amotion vector, a transform type, a transform size, information ofwhether or not a primary (first) transform is used, information ofwhether or not a secondary transform is used, a primary transform index,a secondary transform index, information of whether or not a residualsignal is present, a coded block pattern, a coded block flag (CBF), aquantization parameter, a quantization parameter residue, a quantizationmatrix, whether to apply an intra loop filter, an intra loop filtercoefficient, an intra loop filter tab, an intra loop filter shape/form,whether to apply a deblocking filter, a deblocking filter coefficient, adeblocking filter tab, a deblocking filter strength, a deblocking filtershape/form, whether to apply an adaptive sample offset, an adaptivesample offset value, an adaptive sample offset category, an adaptivesample offset type, whether to apply an adaptive loop filter, anadaptive loop filter coefficient, an adaptive loop filter tab, anadaptive loop filter shape/form, a binarization/inverse-binarizationmethod, a context model determining method, a context model updatingmethod, whether to perform a regular mode, whether to perform a bypassmode, a context bin, a bypass bin, a significant coefficient flag, alast significant coefficient flag, a coded flag for a unit of acoefficient group, a position of the last significant coefficient, aflag for whether a value of a coefficient is larger than 1, a flag forwhether a value of a coefficient is larger than 2, a flag for whether avalue of a coefficient is larger than 3, information on a remainingcoefficient value, a sign information, a reconstructed luma sample, areconstructed chroma sample, a residual luma sample, a residual chromasample, a luma transform coefficient, a chroma transform coefficient, aquantized luma level, a quantized chroma level, a transform coefficientlevel scanning method, a size of a motion vector search area at adecoder side, a shape of a motion vector search area at a decoder side,a number of time of a motion vector search at a decoder side,information on a CTU size, information on a minimum block size,information on a maximum block size, information on a maximum blockdepth, information on a minimum block depth, an imagedisplaying/outputting sequence, slice identification information, aslice type, slice partition information, tile identificationinformation, a tile type, tile partition information, tile groupidentification information, a tile group type, tile group partitioninformation, a picture type, a bit depth of an input sample, a bit depthof a reconstruction sample, a bit depth of a residual sample, a bitdepth of a transform coefficient, a bit depth of a quantized level, andinformation on a luma signal or information on a chroma signal may beincluded in the coding parameter.

Herein, signaling the flag or index may mean that a corresponding flagor index is entropy encoded and included in a bitstream by an encoder,and may mean that the corresponding flag or index is entropy decodedfrom a bitstream by a decoder.

When the encoding apparatus 100 performs encoding throughinter-prediction, an encoded current image may be used as a referenceimage for another image that is processed afterwards. Accordingly, theencoding apparatus 100 may reconstruct or decode the encoded currentimage, or store the reconstructed or decoded image as a reference imagein reference picture buffer 190.

A quantized level may be dequantized in the dequantization unit 160, ormay be inverse-transformed in the inverse-transform unit 170. Adequantized or inverse-transformed coefficient or both may be added witha prediction block by the adder 175. By adding the dequantized orinverse-transformed coefficient or both with the prediction block, areconstructed block may be generated. Herein, the dequantized orinverse-transformed coefficient or both may mean a coefficient on whichat least one of dequantization and inverse-transform is performed, andmay mean a reconstructed residual block.

A reconstructed block may pass through the filter unit 180. The filterunit 180 may apply at least one of a deblocking filter, a sampleadaptive offset (SAO), and an adaptive loop filter (ALF) to areconstructed sample, a reconstructed block or a reconstructed image.The filter unit 180 may be called as an in-loop filter.

The deblocking filter may remove block distortion generated inboundaries between blocks. In order to determine whether or not to applya deblocking filter, whether or not to apply a deblocking filter to acurrent block may be determined based samples included in several rowsor columns which are included in the block. When a deblocking filter isapplied to a block, another filter may be applied according to arequired deblocking filtering strength.

In order to compensate an encoding error, a proper offset value may beadded to a sample value by using a sample adaptive offset. The sampleadaptive offset may correct an offset of a deblocked image from anoriginal image by a sample unit. A method of partitioning samples of animage into a predetermined number of regions, determining a region towhich an offset is applied, and applying the offset to the determinedregion, or a method of applying an offset in consideration of edgeinformation on each sample may be used.

The adaptive loop filter may perform filtering based on a comparisonresult of the filtered reconstructed image and the original image.Samples included in an image may be partitioned into predeterminedgroups, a filter to be applied to each group may be determined, anddifferential filtering may be performed for each group. Information ofwhether or not to apply the ALF may be signaled by coding units (CUs),and a form and coefficient of the ALF to be applied to each block mayvary.

The reconstructed block or the reconstructed image having passed throughthe filter unit 180 may be stored in the reference picture buffer 190. Areconstructed block processed by the filter unit 180 may be a part of areference image. That is, a reference image is a reconstructed imagecomposed of reconstructed blocks processed by the filter unit 180. Thestored reference image may be used later in inter prediction or motioncompensation.

FIG. 2 is a block diagram showing a configuration of a decodingapparatus according to an embodiment and to which the present inventionis applied.

A decoding apparatus 200 may a decoder, a video decoding apparatus, oran image decoding apparatus.

Referring to FIG. 2, the decoding apparatus 200 may include an entropydecoding unit 210, a dequantization unit 220, an inverse-transform unit230, an intra-prediction unit 240, a motion compensation unit 250, anadder 225, a filter unit 260, and a reference picture buffer 270.

The decoding apparatus 200 may receive a bitstream output from theencoding apparatus 100. The decoding apparatus 200 may receive abitstream stored in a computer readable recording medium, or may receivea bitstream that is streamed through a wired/wireless transmissionmedium. The decoding apparatus 200 may decode the bitstream by using anintra mode or an inter mode. In addition, the decoding apparatus 200 maygenerate a reconstructed image generated through decoding or a decodedimage, and output the reconstructed image or decoded image.

When a prediction mode used when decoding is an intra mode, a switch maybe switched to an intra. Alternatively, when a prediction mode used whendecoding is an inter mode, a switch may be switched to an inter mode.

The decoding apparatus 200 may obtain a reconstructed residual block bydecoding the input bitstream, and generate a prediction block. When thereconstructed residual block and the prediction block are obtained, thedecoding apparatus 200 may generate a reconstructed block that becomes adecoding target by adding the reconstructed residual block with theprediction block. The decoding target block may be called a currentblock.

The entropy decoding unit 210 may generate symbols by entropy decodingthe bitstream according to a probability distribution. The generatedsymbols may include a symbol of a quantized level form. Herein, anentropy decoding method may be an inverse-process of the entropyencoding method described above.

In order to decode a transform coefficient level (quantized level), theentropy decoding unit 210 may change a one-directional vector formcoefficient into a two-dimensional block form by using a transformcoefficient scanning method.

A quantized level may be dequantized in the dequantization unit 220, orinverse-transformed in the inverse-transform unit 230. The quantizedlevel may be a result of dequantizing or inverse-transforming or both,and may be generated as a reconstructed residual block. Herein, thedequantization unit 220 may apply a quantization matrix to the quantizedlevel.

When an intra mode is used, the intra-prediction unit 240 may generate aprediction block by performing, for the current block, spatialprediction that uses a sample value of a block adjacent to a decodingtarget block and which has been already decoded.

When an inter mode is used, the motion compensation unit 250 maygenerate a prediction block by performing, for the current block, motioncompensation that uses a motion vector and a reference image stored inthe reference picture buffer 270.

The adder 225 may generate a reconstructed block by adding thereconstructed residual block with the prediction block. The filter unit260 may apply at least one of a deblocking filter, a sample adaptiveoffset, and an adaptive loop filter to the reconstructed block orreconstructed image. The filter unit 260 may output the reconstructedimage. The reconstructed block or reconstructed image may be stored inthe reference picture buffer 270 and used when performinginter-prediction. A reconstructed block processed by the filter unit 260may be a part of a reference image. That is, a reference image is areconstructed image composed of reconstructed blocks processed by thefilter unit 260. The stored reference image may be used later in interprediction or motion compensation.

FIG. 3 is a view schematically showing a partition structure of an imagewhen encoding and decoding the image. FIG. 3 schematically shows anexample of partitioning a single unit into a plurality of lower units.

In order to efficiently partition an image, when encoding and decoding,a coding unit (CU) may be used. The coding unit may be used as a basicunit when encoding/decoding the image. In addition, the coding unit maybe used as a unit for distinguishing an intra prediction mode and aninter prediction mode when encoding/decoding the image. The coding unitmay be a basic unit used for prediction, transform, quantization,inverse-transform, dequantization, or an encoding/decoding process of atransform coefficient.

Referring to FIG. 3, an image 300 is sequentially partitioned in alargest coding unit (LCU), and a LCU unit is determined as a partitionstructure. Herein, the LCU may be used in the same meaning as a codingtree unit (CTU). A unit partitioning may mean partitioning a blockassociated with to the unit. In block partition information, informationof a unit depth may be included. Depth information may represent anumber of times or a degree or both in which a unit is partitioned. Asingle unit may be partitioned into a plurality of lower level unitshierarchically associated with depth information based on a treestructure. In other words, a unit and a lower level unit generated bypartitioning the unit may correspond to a node and a child node of thenode, respectively. Each of partitioned lower unit may have depthinformation. Depth information may be information representing a size ofa CU, and may be stored in each CU. Unit depth represents times and/ordegrees related to partitioning a unit. Therefore, partitioninginformation of a lower-level unit may comprise information on a size ofthe lower-level unit.

A partition structure may mean a distribution of a coding unit (CU)within an LCU 310. Such a distribution may be determined according towhether or not to partition a single CU into a plurality (positiveinteger equal to or greater than 2 including 2, 4, 8, 16, etc.) of CUs.A horizontal size and a vertical size of the CU generated bypartitioning may respectively be half of a horizontal size and avertical size of the CU before partitioning, or may respectively havesizes smaller than a horizontal size and a vertical size beforepartitioning according to a number of times of partitioning. The CU maybe recursively partitioned into a plurality of CUs. By the recursivepartitioning, at least one among a height and a width of a CU afterpartitioning may decrease comparing with at least one among a height anda width of a CU before partitioning. Partitioning of the CU may berecursively performed until to a predefined depth or predefined size.For example, a depth of an LCU may be 0, and a depth of a smallestcoding unit (SCU) may be a predefined maximum depth. Herein, the LCU maybe a coding unit having a maximum coding unit size, and the SCU may be acoding unit having a minimum coding unit size as described above.Partitioning is started from the LCU 310, a CU depth increases by 1 as ahorizontal size or a vertical size or both of the CU decreases bypartitioning. For example, for each depth, a CU which is not partitionedmay have a size of 2N×2N. Also, in case of a CU which is partitioned, aCU with a size of 2N×2N may be partitioned into four CUs with a size ofN×N. A size of N may decrease to half as a depth increase by 1.

In addition, information whether or not the CU is partitioned may berepresented by using partition information of the CU. The partitioninformation may be 1-bit information. All CUs, except for a SCU, mayinclude partition information. For example, when a value of partitioninformation is a first value, the CU may not be partitioned, when avalue of partition information is a second value, the CU may bepartitioned

Referring to FIG. 3, an LCU having a depth 0 may be a 64×64 block. 0 maybe a minimum depth. A SCU having a depth 3 may be an 8×8 block. 3 may bea maximum depth. A CU of a 32×32 block and a 16×16 block may berespectively represented as a depth 1 and a depth 2.

For example, when a single coding unit is partitioned into four codingunits, a horizontal size and a vertical size of the four partitionedcoding units may be a half size of a horizontal and vertical size of theCU before being partitioned. In one embodiment, when a coding unithaving a 32×32 size is partitioned into four coding units, each of thefour partitioned coding units may have a 16×16 size. When a singlecoding unit is partitioned into four coding units, it may be called thatthe coding unit may be partitioned into a quad-tree form.

For example, when one coding unit is partitioned into two sub-codingunits, the horizontal or vertical size (width or height) of each of thetwo sub-coding units may be half the horizontal or vertical size of theoriginal coding unit. For example, when a coding unit having a size of32×32 is vertically partitioned into two sub-coding units, each of thetwo sub-coding units may have a size of 16×32. For example, when acoding unit having a size of 8×32 is horizontally partitioned into twosub-coding units, each of the two sub-coding units may have a size of8×16. When one coding unit is partitioned into two sub-coding units, itcan be said that the coding unit is binary-partitioned or is partitionedby a binary tree partition structure.

For example, when one coding unit is partitioned into three sub-codingunits, the horizontal or vertical size of the coding unit can bepartitioned with a ratio of 1:2:1, thereby producing three sub-codingunits whose horizontal or vertical sizes are in a ratio of 1:2:1. Forexample, when a coding unit having a size of 16×32 is horizontallypartitioned into three sub-coding units, the three sub-coding units mayhave sizes of 16×8, 16×16, and 16×8 respectively, in the order from theuppermost to the lowermost sub-coding unit. For example, when a codingunit having a size of 32×32 is vertically split into three sub-codingunits, the three sub-coding units may have sizes of 8×32, 16×32, and8×32, respectively in the order from the left to the right sub-codingunit. When one coding unit is partitioned into three sub-coding units,it can be said that the coding unit is temary-partitioned or partitionedby a ternary tree partition structure.

In FIG. 3, a coding tree unit (CTU) 320 is an example of a CTU to whicha quad tree partition structure, a binary tree partition structure, anda ternary tree partition structure are all applied.

As described above, in order to partition the CTU, at least one of aquad tree partition structure, a binary tree partition structure, and aternary tree partition structure may be applied. Various tree partitionstructures may be sequentially applied to the CTU, according to apredetermined priority order. For example, the quad tree partitionstructure may be preferentially applied to the CTU. A coding unit thatcannot be partitioned any longer using a quad tree partition structuremay correspond to a leaf node of a quad tree. A coding unitcorresponding to a leaf node of a quad tree may serve as a root node ofa binary and/or ternary tree partition structure. That is, a coding unitcorresponding to a leaf node of a quad tree may be further partitionedby a binary tree partition structure or a ternary tree partitionstructure, or may not be further partitioned. Therefore, by preventing acoding unit that results from binary tree partitioning or ternary treepartitioning of a coding unit corresponding to a leaf node of a quadtree from undergoing further quad tree partitioning, block partitioningand/or signaling of partition information can be effectively performed.

The fact that a coding unit corresponding to a node of a quad tree ispartitioned may be signaled using quad partition information. The quadpartition information having a first value (e.g., “1”) may indicate thata current coding unit is partitioned by the quad tree partitionstructure. The quad partition information having a second value (e.g.,“0”) may indicate that a current coding unit is not partitioned by thequad tree partition structure. The quad partition information may be aflag having a predetermined length (e.g., one bit).

There may not be a priority between the binary tree partitioning and theternary tree partitioning. That is, a coding unit corresponding to aleaf node of a quad tree may further undergo arbitrary partitioningamong the binary tree partitioning and the ternary tree partitioning. Inaddition, a coding unit generated through the binary tree partitioningor the ternary tree partitioning may undergo a further binary treepartitioning or a further ternary tree partitioning, or may not befurther partitioned.

A tree structure in which there is no priority among the binary treepartitioning and the ternary tree partitioning is referred to as amulti-type tree structure. A coding unit corresponding to a leaf node ofa quad tree may serve as a root node of a multi-type tree. Whether topartition a coding unit which corresponds to a node of a multi-type treemay be signaled using at least one of multi-type tree partitionindication information, partition direction information, and partitiontree information. For partitioning of a coding unit corresponding to anode of a multi-type tree, the multi-type tree partition 1 o indicationinformation, the partition direction, and the partition tree informationmay be sequentially signaled.

The multi-type tree partition indication information having a firstvalue (e.g., “1”) may indicate that a current coding unit is to undergoa multi-type tree partitioning. The multi-type tree partition indicationinformation having a second value (e.g., “0”) may indicate that acurrent coding unit is not to undergo a multi-type tree partitioning.

When a coding unit corresponding to a node of a multi-type tree isfurther partitioned by a multi-type tree partition structure, the codingunit may include partition direction information. The partitiondirection information may indicate in which direction a current codingunit is to be partitioned for the multi-type tree partitioning. Thepartition direction information having a first value (e.g., “1”) mayindicate that a current coding unit is to be vertically partitioned. Thepartition direction information having a second value (e.g., “0”) mayindicate that a current coding unit is to be horizontally partitioned.

When a coding unit corresponding to a node of a multi-type tree isfurther partitioned by a multi-type tree partition structure, thecurrent coding unit may include partition tree information. Thepartition tree information may indicate a tree partition structure whichis to be used for partitioning of a node of a multi-type tree. Thepartition tree information having a first value (e.g., “1”) may indicatethat a current coding unit is to be partitioned by a binary treepartition structure. The partition tree information having a secondvalue (e.g., “0”) may indicate that a current coding unit is to bepartitioned by a ternary tree partition structure.

The partition indication information, the partition tree information,and the partition direction information may each be a flag having apredetermined length (e.g., one bit).

At least any one of the quadtree partition indication information, themulti-type tree partition indication information, the partitiondirection information, and the partition tree information may be entropyencoded/decoded. For the entropy-encoding/decoding of those types ofinformation, information on a neighboring coding unit adjacent to thecurrent coding unit may be used. For example, there is a highprobability that the partition type (the partitioned or non-partitioned,the partition tree, and/or the partition direction) of a leftneighboring coding unit and/or an upper neighboring coding unit of acurrent coding unit is similar to that of the current coding unit.Therefore, context information for entropy encoding/decoding of theinformation on the current coding unit may be derived from theinformation on the neighboring coding units. The information on theneighboring coding units may include at least any one of quad partitioninformation, multi-type tree partition indication information, partitiondirection information, and partition tree information.

As another example, among binary tree partitioning and ternary treepartitioning, binary tree partitioning may be preferentially performed.That is, a current coding unit may primarily undergo binary treepartitioning, and then a coding unit corresponding to a leaf node of abinary tree may be set as a root node for ternary tree partitioning. Inthis case, neither quad tree partitioning nor binary tree partitioningmay not be performed on the coding unit corresponding to a node of aternary tree.

A coding unit that cannot be partitioned by a quad tree partitionstructure, a binary tree partition structure, and/or a ternary treepartition structure becomes a basic unit for coding, prediction and/ortransformation. That is, the coding unit cannot be further partitionedfor prediction and/or transformation. Therefore, the partition structureinformation and the partition information used for partitioning a codingunit into prediction units and/or transformation units may not bepresent in a bit stream.

However, when the size of a coding unit (i.e., a basic unit forpartitioning) is larger than the size of a maximum transformation block,the coding unit may be recursively partitioned until the size of thecoding unit is reduced to be equal to or smaller than the size of themaximum transformation block. For example, when the size of a codingunit is 64×64 and when the size of a maximum transformation block is32×32, the coding unit may be partitioned into four 32×32 blocks fortransformation. For example, when the size of a coding unit is 32×64 andthe size of a maximum transformation block is 32×32, the coding unit maybe partitioned into two 32×32 blocks for the transformation. In thiscase, the partitioning of the coding unit for transformation is notsignaled separately, and may be determined through comparison betweenthe horizontal or vertical size of the coding unit and the horizontal orvertical size of the maximum transformation block. For example, when thehorizontal size (width) of the coding unit is larger than the horizontalsize (width) of the maximum transformation block, the coding unit may bevertically bisected. For example, when the vertical size (height) of thecoding unit is larger than the vertical size (height) of the maximumtransformation block, the coding unit may be horizontally bisected.

Information of the maximum and/or minimum size of the coding unit andinformation of the maximum and/or minimum size of the transformationblock may be signaled or determined at an upper level of the codingunit. The upper level may be, for example, a sequence level, a picturelevel, a slice level, a tile group level, a tile level, or the like. Forexample, the minimum size of the coding unit may be determined to be4×4. For example, the maximum size of the transformation block may bedetermined to be 64×64. For example, the minimum size of thetransformation block may be determined to be 4×4.

Information of the minimum size (quad tree minimum size) of a codingunit corresponding to a leaf node of a quad tree and/or information ofthe maximum depth (the maximum tree depth of a multi-type tree) from aroot node to a leaf node of the multi-type tree may be signaled ordetermined at an upper level of the coding unit. For example, the upperlevel may be a sequence level, a picture level, a slice level, a tilegroup level, a tile level, or the like. Information of the minimum sizeof a quad tree and/or information of the maximum depth of a multi-typetree may be signaled or determined for each of an intra-picture sliceand an inter-picture slice.

Difference information between the size of a CTU and the maximum size ofa transformation block may be signaled or determined at an upper levelof the coding unit. For example, the upper level may be a sequencelevel, a picture level, a slice level, a tile group level, a tile level,or the like. Information of the maximum size of the coding unitscorresponding to the respective nodes of a binary tree (hereinafter,referred to as a maximum size of a binary tree) may be determined basedon the size of the coding tree unit and the difference information. Themaximum size of the coding units corresponding to the respective nodesof a ternary tree (hereinafter, referred to as a maximum size of aternary tree) may vary depending on the type of slice. For example, foran intra-picture slice, the maximum size of a ternary tree may be 32×32.For example, for an inter-picture slice, the maximum size of a ternarytree may be 128×128. For example, the minimum size of the coding unitscorresponding to the respective nodes of a binary tree (hereinafter,referred to as a minimum size of a binary tree) and/or the minimum sizeof the coding units corresponding to the respective nodes of a ternarytree (hereinafter, referred to as a minimum size of a ternary tree) maybe set as the minimum size of a coding block.

As another example, the maximum size of a binary tree and/or the maximumsize of a ternary tree may be signaled or determined at the slice level.Alternatively, the minimum size of the binary tree and/or the minimumsize of the ternary tree may be signaled or determined at the slicelevel.

Depending on size and depth information of the above-described variousblocks, quad partition information, multi-type tree partition indicationinformation, partition tree information and/or partition directioninformation may be included or may not be included in a bit stream.

For example, when the size of the coding unit is not larger than theminimum size of a quad tree, the coding unit does not contain quadpartition information. Thus, the quad partition information may beinferred to be a second value.

For example, when the sizes (horizontal and vertical sizes) of a codingunit corresponding to a node of a multi-type tree are larger than themaximum sizes (horizontal and vertical sizes) of a binary tree and/orthe maximum sizes (horizontal and vertical sizes) of a ternary tree, thecoding unit may not be binary-partitioned or ternary-partitioned.Accordingly, the multi-type tree partition indication information maynot be signaled but may be inferred to be a second value.

Alternatively, when the sizes (horizontal and vertical sizes) of acoding unit corresponding to a node of a multi-type tree are the same asthe maximum sizes (horizontal and vertical sizes) of a binary treeand/or are two times as large as the maximum sizes (horizontal andvertical sizes) of a ternary tree, the coding unit may not be furtherbinary-partitioned or ternary-partitioned. Accordingly, the multi-typetree partition indication information may not be signaled but be derivedfrom a second value. This is because when a coding unit is partitionedby a binary tree partition structure and/or a ternary tree partitionstructure, a coding unit smaller than the minimum size of a binary treeand/or the minimum size of a ternary tree is generated.

Alternatively, the binary tree partitioning or the ternary treepartitioning may be limited on the basis of the size of a virtualpipeline data unit (hereinafter, a pipeline buffer size). For example,when the coding unit is divided into sub-coding units which do not fitthe pipeline buffer size by the binary tree partitioning or the ternarytree partitioning, the corresponding binary tree partitioning or ternarytree partitioning may be limited. The pipeline buffer size may be thesize of the maximum transform block (e.g., 64×64). For example, when thepipeline buffer size is 64×64, the division below may be limited.

-   -   N×M (N and/or M is 128) Ternary tree partitioning for coding        units    -   128×N (N<=64) Binary tree partitioning in horizontal direction        for coding units    -   N×128 (N<=64) Binary tree partitioning in vertical direction for        coding units

Alternatively, when the depth of a coding unit corresponding to a nodeof a multi-type tree is equal to the maximum depth of the multi-typetree, the coding unit may not be further binary-partitioned and/orternary-partitioned. Accordingly, the multi-type tree partitionindication information may not be signaled but may be inferred to be asecond value.

Alternatively, only when at least one of vertical direction binary treepartitioning, horizontal direction binary tree partitioning, verticaldirection ternary tree partitioning, and horizontal direction ternarytree partitioning is possible for a coding unit corresponding to a nodeof a multi-type tree, the multi-type tree partition indicationinformation may be signaled. Otherwise, the coding unit may not bebinary-partitioned and/or ternary-partitioned. Accordingly, themulti-type tree partition indication information may not be signaled butmay be inferred to be a second value.

Alternatively, only when both of the vertical direction binary treepartitioning and the horizontal direction binary tree partitioning orboth of the vertical direction ternary tree partitioning and thehorizontal direction ternary tree partitioning are possible for a codingunit corresponding to a node of a multi-type tree, the partitiondirection information may be signaled. Otherwise, the partitiondirection information may not be signaled but may be derived from avalue indicating possible partitioning directions.

Alternatively, only when both of the vertical direction binary treepartitioning and the vertical direction ternary tree partitioning orboth of the horizontal direction binary tree partitioning and thehorizontal direction ternary tree partitioning are possible for a codingtree corresponding to a node of a multi-type tree, the partition treeinformation may be signaled. Otherwise, the partition tree informationmay not be signaled but be inferred to be a value indicating a possiblepartitioning tree structure.

FIG. 4 is a view showing an intra-prediction process.

Arrows from center to outside in FIG. 4 may represent predictiondirections of intra prediction modes.

Intra encoding and/or decoding may be performed by using a referencesample of a neighbor block of the current block. A neighbor block may bea reconstructed neighbor block. For example, intra encoding and/ordecoding may be performed by using a coding parameter or a value of areference sample included in a reconstructed neighbor block.

A prediction block may mean a block generated by performing intraprediction. A prediction block may correspond to at least one among CU,PU and TU. A unit of a prediction block may have a size of one among CU,PU and TU. A prediction block may be a square block having a size of2×2, 4×4, 16×16, 32×32 or 64×64 etc. or may be a rectangular blockhaving a size of 2×8, 4×8, 2×16, 4×16 and 8×16 etc.

Intra prediction may be performed according to intra prediction mode forthe current block. The number of intra prediction modes which thecurrent block may have may be a fixed value and may be a valuedetermined differently according to an attribute of a prediction block.For example, an attribute of a prediction block may comprise a size of aprediction block and a shape of a prediction block, etc.

The number of intra-prediction modes may be fixed to N regardless of ablock size. Or, the number of intra prediction modes may be 3, 5, 9, 17,34, 35, 36, 65, or 67 etc. Alternatively, the number of intra-predictionmodes may vary according to a block size or a color component type orboth. For example, the number of intra prediction modes may varyaccording to whether the color component is a luma signal or a chromasignal. For example, as a block size becomes large, a number ofintra-prediction modes may increase. Alternatively, a number ofintra-prediction modes of a luma component block may be larger than anumber of intra-prediction modes of a chroma component block.

An intra-prediction mode may be a non-angular mode or an angular mode.The non-angular mode may be a DC mode or a planar mode, and the angularmode may be a prediction mode having a specific direction or angle. Theintra-prediction mode may be expressed by at least one of a mode number,a mode value, a mode numeral, a mode angle, and mode direction. A numberof intra-prediction modes may be M, which is larger than 1, includingthe non-angular and the angular mode. In order to intra-predict acurrent block, a step of determining whether or not samples included ina reconstructed neighbor block may be used as reference samples of thecurrent block may be performed. When a sample that is not usable as areference sample of the current block is present, a value obtained byduplicating or performing interpolation on at least one sample valueamong samples included in the reconstructed neighbor block or both maybe used to replace with a non-usable sample value of a sample, thus thereplaced sample value is used as a reference sample of the currentblock.

FIG. 7 is a diagram illustrating reference samples capable of being usedfor intra prediction.

As shown in FIG. 7, at least one of the reference sample line 0 to thereference sample line 3 may be used for intra prediction of the currentblock. In FIG. 7, the samples of a segment A and a segment F may bepadded with the samples closest to a segment B and a segment E,respectively, instead of retrieving from the reconstructed neighboringblock. Index information indicating the reference sample line to be usedfor intra prediction of the current block may be signaled. For example,in FIG. 7, reference sample line indicators 0, 1, and 2 may be signaledas index information indicating reference sample lines 0, 1 and 2. Whenthe upper boundary of the current block is the boundary of the CTU, onlythe reference sample line 0 may be available. Therefore, in this case,the index information may not be signaled. When a reference sample lineother than the reference sample line 0 is used, filtering for aprediction block, which will be described later, may not be performed.

When intra-predicting, a filter may be applied to at least one of areference sample and a prediction sample based on an intra-predictionmode and a current block size.

In case of a planar mode, when generating a prediction block of acurrent block, according to a position of a prediction target samplewithin a prediction block, a sample value of the prediction targetsample may be generated by using a weighted sum of an upper and leftside reference sample of a current block, and a right upper side andleft lower side reference sample of the current block. In addition, incase of a DC mode, when generating a prediction block of a currentblock, an average value of upper side and left side reference samples ofthe current block may be used. In addition, in case of an angular mode,a prediction block may be generated by using an upper side, a left side,a right upper side, and/or a left lower side reference sample of thecurrent block. In order to generate a prediction sample value,interpolation of a real number unit may be performed.

In the case of intra prediction between color components, a predictionblock for the current block of the second color component may begenerated on the basis of the corresponding reconstructed block of thefirst color component. For example, the first color component may be aluma component, and the second color component may be a chromacomponent. For intra prediction between color components, the parametersof the linear model between the first color component and the secondcolor component may be derived on the basis of the template. Thetemplate may include upper and/or left neighboring samples of thecurrent block and upper and/or left neighboring samples of thereconstructed block of the first color component corresponding thereto.For example, the parameters of the linear model may be derived using asample value of a first color component having a maximum value amongsamples in a template and a sample value of a second color componentcorresponding thereto, and a sample value of a first color componenthaving a minimum value among samples in the template and a sample valueof a second color component corresponding thereto. When the parametersof the linear model are derived, a corresponding reconstructed block maybe applied to the linear model to generate a prediction block for thecurrent block. According to a video format, subsampling may be performedon the neighboring samples of the reconstructed block of the first colorcomponent and the corresponding reconstructed block. For example, whenone sample of the second color component corresponds to four samples ofthe first color component, four samples of the first color component maybe sub-sampled to compute one corresponding sample. In this case, theparameter derivation of the linear model and intra prediction betweencolor components may be performed on the basis of the correspondingsub-sampled samples. Whether or not to perform intra prediction betweencolor components and/or the range of the template may be signaled as theintra prediction mode.

The current block may be partitioned into two or four sub-blocks in thehorizontal or vertical direction. The partitioned sub-blocks may besequentially reconstructed. That is, the intra prediction may beperformed on the sub-block to generate the sub-prediction block. Inaddition, dequantization and/or inverse transform may be performed onthe sub-blocks to generate sub-residual blocks. A reconstructedsub-block may be generated by adding the sub-prediction block to thesub-residual block. The reconstructed sub-block may be used as areference sample for intra prediction of the sub-sub-blocks. Thesub-block may be a block including a predetermined number (for example,16) or more samples. Accordingly, for example, when the current block isan 8×4 block or a 4×8 block, the current block may be partitioned intotwo sub-blocks. Also, when the current block is a 4×4 block, the currentblock may not be partitioned into sub-blocks. When the current block hasother sizes, the current block may be partitioned into four sub-blocks.Information on whether or not to perform the intra prediction based onthe sub-blocks and/or the partitioning direction (horizontal orvertical) may be signaled. The intra prediction based on the sub-blocksmay be limited to be performed only when reference sample line 0 isused. When the intra prediction based on the sub-block is performed,filtering for the prediction block, which will be described later, maynot be performed.

The final prediction block may be generated by performing filtering onthe prediction block that is intra-predicted. The filtering may beperformed by applying predetermined weights to the filtering targetsample, the left reference sample, the upper reference sample, and/orthe upper left reference sample. The weight and/or the reference sample(range, position, etc.) used for the filtering may be determined on thebasis of at least one of a block size, an intra prediction mode, and aposition of the filtering target sample in the prediction block. Thefiltering may be performed only in the case of a predetermined intraprediction mode (e.g., DC, planar, vertical, horizontal, diagonal,and/or adjacent diagonal modes). The adjacent diagonal mode may be amode in which k is added to or subtracted from the diagonal mode. Forexample, k may be a positive integer of 8 or less.

An intra-prediction mode of a current block may be entropyencoded/decoded by predicting an intra-prediction mode of a blockpresent adjacent to the current block. When intra-prediction modes ofthe current block and the neighbor block are identical, information thatthe intra-prediction modes of the current block and the neighbor blockare identical may be signaled by using predetermined flag information.In addition, indicator information of an intra-prediction mode that isidentical to the intra-prediction mode of the current block amongintra-prediction modes of a plurality of neighbor blocks may besignaled. When intra-prediction modes of the current block and theneighbor block are different, intra-prediction mode information of thecurrent block may be entropy encoded/decoded by performing entropyencoding/decoding based on the intra-prediction mode of the neighborblock.

FIG. 5 is a diagram illustrating an embodiment of an inter-pictureprediction process.

In FIG. 5, a rectangle may represent a picture. In FIG. 5, an arrowrepresents a prediction direction. Pictures may be categorized intointra pictures (I pictures), predictive pictures (P pictures), andB1-predictive pictures (B pictures) according to the encoding typethereof.

The I picture may be encoded through intra-prediction without requiringinter-picture prediction. The P picture may be encoded throughinter-picture prediction by using a reference picture that is present inone direction (i.e., forward direction or backward direction) withrespect to a current block. The B picture may be encoded throughinter-picture prediction by using reference pictures that are preset intwo directions (i.e., forward direction and backward direction) withrespect to a current block. When the inter-picture prediction is used,the encoder may perform inter-picture prediction or motion compensationand the decoder may perform the corresponding motion compensation.

Hereinbelow, an embodiment of the inter-picture prediction will bedescribed in detail.

The inter-picture prediction or motion compensation may be performedusing a reference picture and motion information.

Motion information of a current block may be derived duringinter-picture prediction by each of the encoding apparatus 100 and thedecoding apparatus 200. The motion information of the current block maybe derived by using motion information of a reconstructed neighboringblock, motion information of a collocated block (also referred to as acol block or a co-located block), and/or a block adjacent to theco-located block. The co-located block may mean a block that is locatedspatially at the same position as the current block, within a previouslyreconstructed collocated picture (also referred to as a col picture or aco-located picture). The co-located picture may be one picture among oneor more reference pictures included in a reference picture list.

The derivation method of the motion information may be differentdepending on the prediction mode of the current block. For example, aprediction mode applied for inter prediction includes an AMVP mode, amerge mode, a skip mode, a merge mode with a motion vector difference, asubblock merge mode, a geometric partitioning mode, an combined interintra prediction mode, affine mode, and the like. Herein, the merge modemay be referred to as a motion merge mode.

For example, when the AMVP is used as the prediction mode, at least oneof motion vectors of the reconstructed neighboring blocks, motionvectors of the co-located blocks, motion vectors of blocks adjacent tothe co-located blocks, and a (0, 0) motion vector may be determined asmotion vector candidates for the current block, and a motion vectorcandidate list is generated by using the emotion vector candidates. Themotion vector candidate of the current block can be derived by using thegenerated motion vector candidate list. The motion information of thecurrent block may be determined based on the derived motion vectorcandidate. The motion vectors of the collocated blocks or the motionvectors of the blocks adjacent to the collocated blocks may be referredto as temporal motion vector candidates, and the motion vectors of thereconstructed neighboring blocks may be referred to as spatial motionvector candidates.

The encoding apparatus 100 may calculate a motion vector difference(MVD) between the motion vector of the current block and the motionvector candidate and may perform entropy encoding on the motion vectordifference (MVD). In addition, the encoding apparatus 100 may performentropy encoding on a motion vector candidate index and generate abitstream. The motion vector candidate index may indicate an optimummotion vector candidate among the motion vector candidates included inthe motion vector candidate list. The decoding apparatus may performentropy decoding on the motion vector candidate index included in thebitstream and may select a motion vector candidate of a decoding targetblock from among the motion vector candidates included in the motionvector candidate list by using the entropy-decoded motion vectorcandidate index. In addition, the decoding apparatus 200 may add theentropy-decoded MVD and the motion vector candidate extracted throughthe entropy decoding, thereby deriving the motion vector of the decodingtarget block.

Meanwhile, the coding apparatus 100 may perform entropy-coding onresolution information of the calculated MVD. The decoding apparatus 200may adjust the resolution of the entropy-decoded MVD using the MVDresolution information.

Meanwhile, the coding apparatus 100 calculates a motion vectordifference (MVD) between a motion vector and a motion vector candidatein the current block on the basis of an affine model, and performsentropy-coding on the MVD. The decoding apparatus 200 derives a motionvector on a per sub-block basis by deriving an affine control motionvector of a decoding target block through the sum of the entropy-decodedMVD and an affine control motion vector candidate.

The bitstream may include a reference picture index indicating areference picture. The reference picture index may be entropy-encoded bythe encoding apparatus 100 and then signaled as a bitstream to thedecoding apparatus 200. The decoding apparatus 200 may generate aprediction block of the decoding target block based on the derivedmotion vector and the reference picture index information.

Another example of the method of deriving the motion information of thecurrent may be the merge mode. The merge mode may mean a method ofmerging motion of a plurality of blocks. The merge mode may mean a modeof deriving the motion information of the current block from the motioninformation of the neighboring blocks. When the merge mode is applied,the merge candidate list may be generated using the motion informationof the reconstructed neighboring blocks and/or the motion information ofthe collocated blocks. The motion information may include at least oneof a motion vector, a reference picture index, and an inter-pictureprediction indicator. The prediction indicator may indicateone-direction prediction (L0 prediction or L1 prediction) ortwo-direction predictions (L0 prediction and L1 prediction).

The merge candidate list may be a list of motion information stored. Themotion information included in the merge candidate list may be at leastone of motion information (spatial merge candidate) of a neighboringblock adjacent to the current block, motion information (temporal mergecandidate) of the collocated block of the current block in the referencepicture, new motion information generated by a combination of the motioninformation exiting in the merge candidate list, motion information(history-based merge candidate) of the block that is encoded/decodedbefore the current block, and zero merge candidate.

The encoding apparatus 100 may generate a bitstream by performingentropy encoding on at least one of a merge flag and a merge index andmay signal the bitstream to the decoding apparatus 200. The merge flagmay be information indicating whether or not to perform the merge modefor each block, and the merge index may be information indicating thatwhich neighboring block, among the neighboring blocks of the currentblock, is a merge target block. For example, the neighboring blocks ofthe current block may include a left neighboring block on the left ofthe current block, an upper neighboring block disposed above the currentblock, and a temporal neighboring block temporally adjacent to thecurrent block.

Meanwhile, the coding apparatus 100 performs entropy-coding on thecorrection information for correcting the motion vector among the motioninformation of the merge candidate and signals the same to the decodingapparatus 200. The decoding apparatus 200 can correct the motion vectorof the merge candidate selected by the merge index on the basis of thecorrection information. Here, the correction information may include atleast one of information on whether or not to perform the correction,correction direction information, and correction size information. Asdescribed above, the prediction mode that corrects the motion vector ofthe merge candidate on the basis of the signaled correction informationmay be referred to as a merge mode having the motion vector difference.

The skip mode may be a mode in which the motion information of theneighboring block is applied to the current block as it is. When theskip mode is applied, the encoding apparatus 100 may perform entropyencoding on information of the fact that the motion information of whichblock is to be used as the motion information of the current block togenerate a bit stream, and may signal the bitstream to the decodingapparatus 200. The encoding apparatus 100 may not signal a syntaxelement regarding at least any one of the motion vector differenceinformation, the encoding block flag, and the transform coefficientlevel to the decoding apparatus 200.

The subblock merge mode may mean a mode that derives the motioninformation in units of sub-blocks of a coding block (CU). When thesubblock merge mode is applied, a subblock merge candidate list may begenerated using motion information (sub-block based temporal mergecandidate) of the sub-block collocated to the current sub-block in thereference image and/or an affine control point motion vector mergecandidate.

The geometric partitioning mode may mean a mode that derives motioninformation by partitioning the current block into the predefineddirections, derives each prediction sample using each of the derivedmotion information, and derives the prediction sample of the currentblock by weighting each of the derived prediction samples.

The inter-intra combined prediction mode may mean a mode that derives aprediction sample of the current block by weighting a prediction samplegenerated by inter prediction and a prediction sample generated by intraprediction.

The decoding apparatus 200 may correct the derived motion information byitself. The decoding apparatus 200 may search the predetermined regionon the basis of the reference block indicated by the derived motioninformation and derive the motion information having the minimum SAD asthe corrected motion information.

The decoding apparatus 200 may compensate a prediction sample derivedvia inter prediction using an optical flow.

FIG. 6 is a diagram illustrating a transform and quantization process.

As illustrated in FIG. 6, a transform and/or quantization process isperformed on a residual signal to generate a quantized level signal. Theresidual signal is a difference between an original block and aprediction block (i.e., an intra prediction block or an inter predictionblock). The prediction block is a block generated through intraprediction or inter prediction. The transform may be a primarytransform, a secondary transform, or both. The primary transform of theresidual signal results in transform coefficients, and the secondarytransform of the transform coefficients results in secondary transformcoefficients.

At least one scheme selected from among various transform schemes whichare preliminarily defined is used to perform the primary transform. Forexample, examples of the predefined transform schemes include discretecosine transform (DCT), discrete sine transform (DST), andKarhunen-Loéve transform (KLT). The transform coefficients generatedthrough the primary transform may undergo the secondary transform. Thetransform schemes used for the primary transform and/or the secondarytransform may be determined according to coding parameters of thecurrent block and/or neighboring blocks of the current block.Alternatively, transform information indicating the transform scheme maybe signaled. The DCT-based transform may include, for example, DCT-2,DCT-8, and the like. The DST-based transform may include, for example,DST-7.

A quantized-level signal (quantization coefficients) may be generated byperforming quantization on the residual signal or a result of performingthe primary transform and/or the secondary transform. The quantizedlevel signal may be scanned according to at least one of a diagonalup-right scan, a vertical scan, and a horizontal scan, depending on anintra prediction mode of a block or a block size/shape. For example, asthe coefficients are scanned in a diagonal up-right scan, thecoefficients in a block form change into a one-dimensional vector form.Aside from the diagonal up-right scan, the horizontal scan ofhorizontally scanning a two-dimensional block form of coefficients orthe vertical scan of vertically scanning a two-dimensional block form ofcoefficients may be used depending on the intra prediction mode and/orthe size of a transform block. The scanned quantized-level coefficientsmay be entropy-encoded to be inserted into a bitstream.

A decoder entropy-decodes the bitstream to obtain the quantized-levelcoefficients. The quantized-level coefficients may be arranged in atwo-dimensional block form through inverse scanning. For the inversescanning, at least one of a diagonal up-right scan, a vertical scan, anda horizontal scan may be used.

The quantized-level coefficients may then be dequantized, then besecondary-inverse-transformed as necessary, and finally beprimary-inverse-transformed as necessary to generate a reconstructedresidual signal.

Inverse mapping in a dynamic range may be performed for a luma componentreconstructed through intra prediction or inter prediction beforein-loop filtering. The dynamic range may be divided into 16 equal piecesand the mapping function for each piece may be signaled. The mappingfunction may be signaled at a slice level or a tile group level. Aninverse mapping function for performing the inverse mapping may bederived on the basis of the mapping function. In-loop filtering,reference picture storage, and motion compensation are performed in aninverse mapped region, and a prediction block generated through interprediction is converted into a mapped region via mapping using themapping function, and then used for generating the reconstructed block.However, since the intra prediction is performed in the mapped region,the prediction block generated via the intra prediction may be used forgenerating the reconstructed block without mapping/inverse mapping.

When the current block is a residual block of a chroma component, theresidual block may be converted into an inverse mapped region byperforming scaling on the chroma component of the mapped region. Theavailability of the scaling may be signaled at the slice level or thetile group level. The scaling may be applied only when the mapping forthe luma component is available and the division of the luma componentand the division of the chroma component follow the same tree structure.The scaling may be performed on the basis of an average of sample valuesof a luma prediction block corresponding to the color difference block.In this case, when the current block uses inter prediction, the lumaprediction block may mean a mapped luma prediction block. A valuenecessary for the scaling may be derived by referring to a lookup tableusing an index of a piece to which an average of sample values of a lumaprediction block belongs. Finally, by scaling the residual block usingthe derived value, the residual block may be switched to the inversemapped region. Then, chroma component block restoration, intraprediction, inter prediction, in-loop filtering, and reference picturestorage may be performed in the inverse mapped area.

Information indicating whether the mapping/inverse mapping of the lumacomponent and chroma component is available may be signaled through aset of sequence parameters.

The prediction block of the current block may be generated on the basisof a block vector indicating a displacement between the current blockand the reference block in the current picture. In this way, aprediction mode for generating a prediction block with reference to thecurrent picture is referred to as an intra block copy (IBC) mode. TheIBC mode may be applied to M×N (M<=64, N<=64) coding units. The IBC modemay include a skip mode, a merge mode, an AMVP mode, and the like. Inthe case of a skip mode or a merge mode, a merge candidate list isconstructed, and the merge index is signaled so that one merge candidatemay be specified. The block vector of the specified merge candidate maybe used as a block vector of the current block. The merge candidate listmay include at least one of a spatial candidate, a history-basedcandidate, a candidate based on an average of two candidates, and azero-merge candidate. In the case of an AMVP mode, the difference blockvector may be signaled. In addition, the prediction block vector may bederived from the left neighboring block and the upper neighboring blockof the current block. The index on which neighboring block to use may besignaled. The prediction block in the IBC mode is included in thecurrent CTU or the left CTU and limited to a block in the alreadyreconstructed area. For example, a value of the block vector may belimited such that the prediction block of the current block ispositioned in an area of three 64×64 blocks preceding the 64×64 block towhich the current block belongs in the coding/decoding order. Bylimiting the value of the block vector in this way, memory consumptionand device complexity according to the IBC mode implementation may bereduced.

Hereinafter, embodiments of the present invention will be described withreference to FIG. 8 to FIG. 20.

FIG. 8 is a flowchart illustrating an image encoding method according toan embodiment of the present invention, and FIG. 9 is a flowchartillustrating an image decoding method according to an embodiment of thepresent invention.

[E1/D1] Deriving a Motion Vector Candidate

Deriving a motion vector candidate [E1/D1] may include at least one ormore among deriving a spatial motion vector candidate [E1-1/D1-1],deriving a temporal motion vector candidate [E1-2/D1-2], and deriving amotion vector candidate based on history [E1-3/D1-3]. Here, when acurrent block is an IBC (Intra Block Copy) mode, a motion vector maymean a block vector.

[E1-1/D1-1] Deriving a Spatial Motion Vector Candidate

An encoder/decoder may derive a motion vector candidate from areconstructed block that is spatially adjacent to an encoding/decodingtarget block.

For example, as shown in FIG. 10, an encoder/decoder may determine aspatial motion vector candidate for an encoding/decoding target block byderiving a spatial motion vector in the block B1 adjacent to the top ofthe encoding/decoding target block X, the block A1 adjacent to the leftof the encoding/decoding target block, the block B0 adjacent to the topright corner of the encoding/decoding target block, the block B2adjacent to the top left corner of the encoding/decoding target block,and the block A0 adjacent to the left bottom corner of theencoding/decoding target block.

In addition, an encoder/decoder may judge whether or not there is amotion vector in each block according to a predetermined order of blocksincluded in positions A0, A1, B0, B1 and B2. In case there is a motionvector, the encoder/decoder may determine the motion vector of acorresponding block as a spatial motion vector candidate.

When an encoding/decoding target block is an IBC (Intra Block Copy) modereferring to a current picture, only a neighbor block that is encodedinto an IBC mode may be determined as a spatial motion vector candidate.Here, a spatial motion vector candidate may be a spatial block vectorcandidate.

When an encoding/decoding target block is an inter prediction mode butnot an IBC mode, if a neighbor block is encoded into an IBC mode, anencoder/decoder may not use the motion vector of a corresponding blockas a spatial motion vector candidate.

In other words, only when a spatial neighbor block has the sameprediction mode as a current block, an encoder/decoder may determine themotion vector of the spatial neighbor block as a spatial motion vectorcandidate.

In addition, when a reference picture of at least one or more blocksincluded in positions A0, A1, B0, B1 and B2 is different from areference picture of an encoding/decoding target block, anencoder/decoder may scale a motion vector of a corresponding block anddetermine it as a spatial motion vector candidate by using a distancebetween an encoding/decoding target picture and a reference picture ofthe corresponding block and a distance between the encoding/decodingtarget picture and a reference picture of the encoding/decoding targetblock. Herein, an encoder/decoder may scale a motion vector based on areference picture index instead of a reference picture.

In addition, when performing the scaling, an encoder/decoder maydetermine a spatial motion vector candidate by scaling at least one ormore motion vectors of blocks included in positions A0, A1, B0, B1 andB2 on the basis of a reference picture corresponding to a referencepicture index with a specific value. Here, the specific value may be apositive integer including 0.

In addition, an encoder/decoder may derive a spatial motion vectorcandidate based on at least one or more coding parameters.

In addition, an encoder/decoder may determine a motion vector of aspatial neighbor block as a spatial motion vector candidate on the basisof the size or area of a current block.

For example, an encoder/decoder may determine a motion vector of aneighbor block as a spatial motion vector candidate only when the areaof a current block is larger than a predefined value.

For another example, if a current block is an IBC mode, anencoder/decoder may determine a block vector of a neighbor block as aspatial block vector candidate, only when the area of the current blockis larger than a predefined value.

Here, the predefined value may be 16.

[E1-2/D1-2] Deriving a Temporal Motion Vector Candidate

An encoder/decoder may derive a motion vector candidate from a blockthat is reconstructed in a co-located picture temporally adjacent to anencoding/decoding target block.

For example, as shown in FIG. 11, an encoder/decoder may derive atemporal motion vector candidate in the order of a block in the positionH, which is outside the co-located block C spatially con-esponding to anencoding/decoding target block X, and a block in the position C3, in aco-located picture of an encoding/decoding target picture. Here, when amotion vector can be derived from a block in the position H, anencoder/decoder may derive a temporal motion vector candidate in theblock located at H. On the other hand, when no motion vector can bederived from the block located at H, the encoder/decoder may derive atemporal motion vector candidate in the position C3. If H or C3 as apredetermined position is intra-encoded or encoded into an IBC (IntraBlock Copy) mode using a current picture as a reference picture, anencoder/decoder may not derive a temporal motion vector candidate. Insuch a case, a temporal motion vector candidate may mean a motion vectorof a co-located block.

In other words, only when a temporal neighbor block has the sameprediction mode as a current block, an encoder/decoder may determine amotion vector of the temporal neighbor block as a temporal motion vectorcandidate.

In addition, an encoder/decoder may derive the temporal motion vectorcandidate based on at least one or more coding parameters.

When a distance between a picture including an encoding/decoding targetblock and a reference block of the encoding/decoding target block isdifferent from a distance between a picture including a co-located blockand a reference picture of the co-located block, an encoder/decoder mayderive a temporal motion vector candidate by scaling a motion vector ofthe co-located block. Herein, an encoder/decoder may scale a motionvector based on a reference picture index instead of a referencepicture.

[E1-3/D1-3] Deriving a History-Based Motion Vector Candidate

At least one piece of information that is used for an encoding/decodingprocess in an encoder/decoder or is generated after theencoding/decoding process may be included in a history-based candidatelist (that is, an HMVP candidate list).

Here, the information of the block may be at least one among codingparameters like an intra prediction mode and motion information.

When the current block is not an affine mode or does not use a temporalmotion vector candidate in sub blocks, at least one piece of blockinformation of the current block may be included in an HMVP candidatelist.

When the current block is an IBC (Intra Block Copy) mode using a currentpicture as a reference picture, it may be included in a separate HMVPcandidate list. Here, a separate candidate list may be an IBC HMVPcandidate list.

Unlike conventional candidate lists configured in block units (motionvector candidate lists and merge candidate lists), the HMVP candidatelist are maintained while being encoded/decoded in units of picture,slice, tile, CTU, CTU row and CTU column. Accordingly, it may be usedwithin units of picture, slice, tile, CTU, CTU row and CTU column. Inaddition, the HMVP candidate list may include at least one among piecesof block information of a block that is encoded/decoded in units ofpicture, slice, tile, CTU, CTU row and CTU column before a currentblock. In addition, the HMVP candidate list may include at least oneamong pieces of block information of a block that is previouslyencoded/decoded in units of picture, slice, tile, CTU, CTU row and CTUcolumn.

As shown in the example of FIG. 12, an encoder/decoder may determine atleast one piece of block information of candidate in an HMVP candidatelist for an encoding/decoding process of a current block. Anencoder/decoder may perform an encoding/decoding process of a currentblock by using at least one piece of block information of a selectedcandidate.

An encoder/decoder may include at least one piece of block informationused during an encoding/decoding process of a current block or at leastone piece of block information used after the encoding/decoding processof the current block in an HMVP candidate list. Here, including at leastone among block information, a candidate and a block in a candidate listmay mean adding at least one among block information, a candidate and ablock to an HMVP candidate list.

When at least one piece of block information of the current block isincluded in an HMVP candidate list, the piece of block information ofthe current block may be added first or last to the HMVP candidate list.

The maximum number of candidates in the HMVP candidate list may bedetermined as P. Here, P may be a positive integer including 0. The Pmay be determined based on at least one of a coding parameter of acurrent block and a coding parameter of a candidate. In addition, the Pmay be a value that is already set in an encoder/decoder or is signaledfrom an encoder to a decoder.

A candidate in the HMVP candidate list may be used to configure at leastone among an intra prediction mode candidate list, a first MPM (MostProbable Mode) list, a second MPM list, a residual intra prediction modecandidate list, a motion vector candidate list, a merge candidate list,and an IBC candidate list.

Among pieces of block information, an intra prediction encoding mode maybe included in the HMVP candidate list. A candidate in the HMVPcandidate list including an intra prediction encoding mode may be usedto configure an intra prediction mode candidate list (for example, afirst MPM list, a second MPM list, a residual mode list, etc.). Thecandidate may be included in an intra prediction mode candidate list.

Among pieces of block information, inter-coding information (forexample, a motion vector, a reference picture index, reference picturelist information, bi-prediction weight information (bcwIdx), ½interpolation filter information (HpelIfIdx) etc.) may be included inthe HMVP candidate list. A candidate in the candidate list includinginter-coding information may be used to configure a motion vectorcandidate list. The candidate may be included in a motion vectorcandidate list.

Among pieces of block information, inter-coding information (forexample, a motion vector, a reference picture index, reference picturelist information, bi-prediction weight information (bcwIdx), ½interpolation filter information (HpelIfIdx) etc.) may be included inthe HMVP candidate list. A candidate in the HMVP candidate listincluding inter-coding information may be used to configure a mergecandidate list. The candidate may be included in a merge candidate list.

In this step, a history-based motion vector candidate list may beconfigured which includes inter-coding information (for example, amotion vector, a reference picture index, reference picture listinformation, bi-prediction weight information (bcwIdx), ½ interpolationfilter information (HpelIfIdx) etc.) among pieces of block information.

[E2/D2] Configuring a Motion Vector Candidate List

Configuring a motion vector candidate list [E2/D2] may includegenerating and adding a combined motion vector candidate to a motionvector candidate list [E2-1/D2-1].

An encoder/decoder may configure a motion vector candidate list eitherby adding a derived motion vector candidate to a motion vector candidatelist or by removing a motion vector candidate included in a motionvector candidate list.

The derived spatial motion vector candidates, temporal motion vectorcandidates, and a history-based motion vector candidate list may beadded to a motion vector candidate list mvpListLX in a predeterminedorder. mvpListLX may mean a motion vector candidate list correspondingto at least one or more reference picture lists like L0, L1, L2 and L3.For example, a motion vector candidate list corresponding to thereference picture list L0 may be expressed as mvpListL0.

Hereinafter, embodiments of configuring a motion vector candidate listwill be described.

For example, an encoder/decoder may sequentially add the derived spatialmotion vector candidates, a temporal motion vector candidate and ahistory-based motion vector candidate list to a motion vector candidatelist.

For example, an encoder/decoder may sequentially add the derived spatialmotion vector candidates, a history-based motion vector candidate list,and a temporal motion vector candidate to a motion vector candidatelist.

For example, an encoder/decoder may add a history-based motion vectorcandidate list among the derived spatial motion vector candidates andthen add a temporal motion vector candidate to a motion vector candidatelist. Specifically, an encoder/decoder may sequentially add N spatialmotion vector candidates, a history-based motion vector candidate list,M spatial motion vector candidates and temporal motion vector candidatesto a motion vector candidate list. Here, N and M may be positiveintegers equal to or greater than 0.

Alternatively, an encoder/decoder may add spatial motion vectorcandidates, temporal motion vector candidates, and a history-basedmotion vector candidate list to a motion vector candidate list in apredetermined order.

When configuring the motion vector candidate list, an encoder/decodermay judge whether or not motion information is identical only amongspatial motion vector candidates. In case the number of candidates in alist does not satisfy the maximum number of motion vector candidates andthus a temporal motion vector candidate or a history-based motion vectorcandidate is added, the encoder/decor may not check redundancy withpreceding candidates.

In case an encoder/decoder adds a history-based motion vector candidatewhile configuring the motion vector candidate list, the encoder/decodermay add information of a maximum number of N candidate blocks in ahistory-based candidate list to the motion vector candidate list. N maybe greater than 0 and be equal to a maximum number of candidates in ahistory-based candidate list.

In case an encoder/decoder adds a history-based motion vector candidatewhile configuring the motion vector candidate list, the encoder/decodermay add information of a maximum number of N candidate blocks in ahistory-based candidate list to the motion vector candidate list. N maybe greater than 0 and be equal to a maximum number of candidates in ahistory-based candidate list. In case the maximum number of candidatesin a history-based candidate list is greater than 4, information of asmany as 4 candidate blocks may be added to a motion vector candidatelist.

In case an encoder/decoder adds a history-based motion vector candidatewhile configuring an IBC candidate list, the encoder/decoder may addinformation of P candidate blocks in a history-based candidate list tothe IBC candidate list. Here, P may be 1, and the motion vectorcandidate may be a block vector candidate.

Apart from the derived spatial motion vector candidate, a history-basedmotion vector candidate list and a temporal motion vector candidate, anencoder/decoder may add a vector with a predetermined value to a motionvector candidate list mvpListLX.

[E2-1/D2-1] Generating and Adding a Combined Motion Vector Candidate toa Motion Vector Candidate List

An encoder/decoder may generate a combined motion vector candidate byusing at least one or more among a spatial motion vector candidate in amotion vector candidate list, a history-based motion vector candidatelist, a temporal motion vector candidate and a zero motion vectorcandidate and may also add the combined motion vector candidate to amotion vector candidate list.

In addition, an encoder/decoder may generate the combined motion vectorcandidate based on at least one or more coding parameters. In addition,an encoder/decoder may add the combined motion vector candidate to amotion vector candidate list, based on at least one or more codingparameters.

[E3/D3] Determining a Predicted Motion Vector from a Motion VectorCandidate List

An encoder/decoder may determine a motion vector candidate in a motionvector candidate list mvpListLX corresponding to a motion vectorcandidate index as a predicted motion vector.

An encoder may work out a motion vector difference by calculating adifference between a motion vector and a predicted motion vector, and adecoder may work out a motion vector by adding a predicted motion vectorand a motion vector difference.

[E4/D4] Performing Motion Compensation

In an encoder/decoder, inter prediction or motion compensation may beperformed by using a motion vector thus determined.

[E5/D5] Entropy Encoding/Decoding of Information about MotionCompensation of a Current Encoding/Decoding Target Block.

An encoder/decoder may entropy encode/decode information of motioncompensation from a bitstream. Here, information of motion compensationmay include at least one among the following pieces of information.

-   -   inter_pred_idc indicating an inter prediction indicator    -   Reference picture indexes (ref_idx_l0, ref_idx_l1, ref_idx_l2,        ref_idx_l3)    -   Motion vector candidate indexes (mvp_l0_flag, mvp_l1_flag,        mvp_l2_flag, mvp_l3_flag)    -   Motion vector difference values    -   cu_skip_flag indicating whether or not a skip mode is used    -   merge_flag indicating whether or not a merge mode is used    -   merge_idx (merge index) indicating a merge candidate    -   Weighting factors (wf_l0, wf_l1, wf_l2, wf_l3)    -   Offset values (offset_l0, offset_l1, offset_l2, offset_l3)

The inter prediction indicator may refer to a direction of interprediction (uni-directional prediction, bi-directional prediction,tri-directional prediction, quad-directional prediction, etc.) of acurrent block during inter prediction. This may mean the number ofreference pictures used by a current block to generate a predictionblock. Alternatively, one reference picture may be used formulti-directional prediction. In this case, M-directional prediction maybe performed by using N reference pictures (N<M). Alternatively, theinter prediction indicator may refer to the number of prediction blocksused by a current block to perform inter prediction or motioncompensation. In addition, it may mean the number of prediction blocksthat a current block uses when performing inter prediction or motioncompensation through at least one or more reference picture lists likeL0, L1, L2 and L3. Here, L0, L1, L2 and L3 may refer to List 0, List 1,List 2 and List 3 respectively. In addition, the inter predictionindicator may be information about whether or not a current block refersto a maximum number of N reference picture lists. Here, N may be 1, 2,3, 4 and above or be a positive integer equal to or greater than 1. Acurrent block may perform motion compensation by using the one or morereference picture lists.

For example, an encoder/decoder may perform motion compensation by usingreference picture lists L0 and L1 and thus generating at least one ormore prediction blocks.

For example, an encoder/decoder may perform motion compensation by usingreference picture lists L0, L1 and L2 and thus generating at least oneprediction block.

For example, an encoder/decoder may perform motion compensation by usingreference picture lists L0, L1 and L2 and thus generating at least oneor more prediction blocks.

For example, an encoder/decoder may perform motion compensation by usingreference picture lists L0, L1 and L2 and thus 1 o generating at leastone or more prediction blocks and a maximum number of N predictionblocks. Here, N may be 3 or be a positive integer equal to or greaterthan 2.

For example, an encoder/decoder may perform motion compensation by usingreference picture lists L0, L1, L2 and L3 and thus generating oneprediction block.

For example, an encoder/decoder may perform motion compensation by usingreference picture lists L0, L1, L2 and L3 and thus generating at leastone or more prediction blocks.

For example, an encoder/decoder may perform motion compensation by usingreference picture lists of L0, L1, L2 and L3 and thus generating atleast one or more prediction blocks and a maximum number of N predictionblocks. Here, N may be 4 or be a positive integer equal to or greaterthan 2.

An available inter prediction direction may be determined based on aninter prediction indicator. Some or all of available inter predictiondirections may be selectively used based on the size and/or shape of acurrent block.

An inter prediction indicator may be information indicating whetherinter prediction is a uni-prediction using L0 (List 0), a uni-predictionusing L1 (List 1) or a bi-prediction using both L0 and L1. The interprediction indicator may be expressed by a prediction list utilizationflag form that includes a flag indicating whether or not L0 directionprediction is used and a flag indicating whether or not L1 directionprediction is used.

Here, a prediction list utilization flag may indicate whether or not aprediction block is generated using a corresponding reference picturelist. For example, when a prediction list utilization flag indicates afirst value (1), it may mean that a corresponding reference picture listis used to generate a prediction block. On the other hand, when theprediction list utilization flag indicates a second value (0), it maymean that a corresponding reference picture list is not used to generatea prediction block. In other words, a prediction block for a currentblock may be generated only by using motion information of a predictionlist utilization flag indicating that a corresponding reference picturelist may be used to generate a prediction block. In addition, only whena prediction list utilization flag has a first value, a prediction blockfor a current block may be generated using the corresponding motioninformation. In addition, a prediction list utilization flag may be setbased on an inter prediction indicator, and an inter predictionindicator may be set based on a prediction list utilization flag.

In addition, num_ref_idx_l0_active_minus1, num_ref_idx_l1_active_minus1,num_ref_idx_l2_active_minus1, and num_ref_idx_l3_active_minus1 may referto the number of reference pictures for reference picture lists L0, L1,L2 and L3 respectively.

The reference picture index may mean a reference picture referred to bya current block in each reference picture list. For each referencepicture list, one or more reference picture indexes may be entropydecoded. An encoder/decoder may perform motion compensation by using theone or more reference picture indexes.

For example, an encoder/decoder may perform motion compensation by usingone reference picture index and thus generating at least one or moreprediction blocks.

For example, an encoder/decoder may perform motion compensation by usingtwo reference picture indexes and thus generating at least one or moreprediction blocks and a maximum number of N prediction blocks. Here, Nmay be 2 or be a positive integer equal to or greater than 2.

For example, an encoder/decoder may perform motion compensation by usingthree reference picture indexes and thus generating at least one or moreprediction blocks and a maximum number of N prediction blocks. Here, Nmay be 3 or be a positive integer equal to or greater than 3.

For example, an encoder/decoder may perform motion compensation by usingfour reference picture indexes and thus generating at least one or moreprediction blocks and a maximum number of N prediction blocks. Here, Nmay be 4 or be a positive integer equal to or greater than 4.

The motion vector candidate index may indicate a motion vector candidateused by a current block in a motion vector candidate list that isgenerated from each reference picture list and/or each reference pictureindex. For each reference picture list and/or each reference pictureindex, one or more motion vector candidate indexes may be entropydecoded. An encoder/decoder may perform motion compensation by using theone or more motion vector candidate indexes.

For example, an encoder/decoder may perform motion compensation by usingone motion vector candidate index and thus generating at least one ormore prediction blocks.

For example, an encoder/decoder may perform motion compensation by usingtwo motion vector candidate indexes and thus generating at least one ormore prediction blocks and a maximum number of N prediction blocks.Here, N may be 2 or be a positive integer equal to or greater than 2.

For example, an encoder/decoder may perform motion compensation by usingthree motion vector candidate indexes and thus generating at least oneor more prediction blocks and a maximum number of N prediction blocks.Here, N may be 3 or be a positive integer equal to or greater than 3.

For example, an encoder/decoder may perform motion compensation by usingfour motion vector candidate indexes and thus generating at least one ormore prediction blocks and a maximum number of N prediction blocks.Here, N may be 4 or be a positive integer equal to or greater than 4.

The motion vector difference may mean a difference value between amotion vector and a predicted motion vector. For a current block, one ormore motion vector differences in each reference picture list and/oreach reference picture index may be entropy decoded. An encoder/decodermay perform motion compensation by using the one or more motion vectordifferences.

For example, an encoder/decoder may perform motion compensation by usingone motion vector difference and thus generating at least one or moreprediction blocks.

For example, an encoder/decoder may perform motion compensation by usingtwo motion vector differences and thus generating at least one or moreprediction blocks and a maximum number of N prediction blocks. Here, Nmay be 2 or be a positive integer equal to or greater than 2.

For example, an encoder/decoder may perform motion compensation by usingthree motion vector differences and thus generating at least one or moreprediction blocks and a maximum number of N prediction blocks. Here, Nmay be 3 or be a positive integer equal to or greater than 3.

For example, an encoder/decoder may perform motion compensation by usingfour motion vector differences and thus generating at least one or moreprediction blocks and a maximum number of N prediction blocks. Here, Nmay be 4 or be a positive integer equal to or greater than 4.

The cu_skip_flag may mean information about whether or not to use a skipmode and may be entropy encoded/decoded in at least one or more units ofan encoding block and a prediction block. For example, when informationabout whether or not to use a skip mode has a first value (1), the useof skip mode may be indicated. When the information about whether or notto use a skip mode has a second value (0), the use of skip mode may notbe indicated.

The merge_flag may mean information about whether or not to use a mergemode and may be entropy encoded/decoded in at least one or more units ofan encoding block and a prediction block. For example, when informationabout whether or not to use a merge mode has a first value (1), the useof merge mode may be indicated. When the information about whether ornot to use a merge mode has a second value (0), the use of merge modemay not be indicated.

The merge_idx may mean information indicating a merge candidate within amerge candidate list and may be entropy encoded/decoded in at least oneor more units of an encoding block and a prediction block. In addition,merge_idx may mean merge index information. In addition, merge_idx mayindicate a block from which a merge candidate is derived, amongreconstructed blocks spatially/temporally adjacent to a current block.In addition, merge_idx may indicate at least one or more pieces ofmotion information of a merge candidate. For example, when merge indexinformation has a first value (0), it may indicate a first mergecandidate within a merge candidate list. When merge index informationhas a second value (1), it may indicate a second merge candidate withina merge candidate list. When merge index information has a third value(2), it may indicate a third merge candidate within a merge candidatelist. Likewise, when it has a fourth or N-th value, it may indicate amerge candidate with a corresponding value according to an order in amerge candidate list. Here, N may be a positive integer including 0.

When motion compensation is performed, at least two or more predictionblocks may be generated and then a weighted sum may be calculated byusing at least one or more among weighting factors and offsets for eachprediction block. The weighted sum thus calculated may be used for interprediction or motion compensation of a current block. At least one ormore of a weighting factor and an offset for the prediction block may beentropy encoded/decoded in at least one or more units of an encodingblock and a prediction block. Herein, at least one or more of aweighting factor and an offset of each prediction block may be entropyencoded/decoded for at 1 o least one or more among a reference picturelist, a reference picture, a motion vector candidate index, a motionvector difference, a motion vector, information about whether or not touse a skip mode, information about whether or not to use a merge mode,and merge index information. In addition, at least one or more of aweighting factor and an offset value of each prediction block may beentropy encoded/decoded based on an inter prediction indicator.

At least one or more among the above-described pieces of informationabout motion compensation may be entropy encoded/decoded in at least oneor more units of CTU and sub-CTU. Here, a sub-CTU may include at leastone or more among a sub-CTU, a CU and a PU.

For example, when at least one or more pieces of information aboutmotion compensation are entropy encoded/decoded in CTUs, motioncompensation may be performed using at least one or more pieces ofinformation about the motion compensation in all the blocks existing inCTUs.

For example, when at least one or more pieces of information aboutmotion compensation are entropy encoded/decoded in CTUs or sub-CTUs, atleast one or more pieces of information about the motion compensation ina specific block size or at a specific block depth may be entropyencoded/decoded. Here, information about a specific block size or aspecific block depth may be additionally entropy encoded/decoded. Inaddition, a block size or a block depth, which is set in anencoder/decoder beforehand, may be used as a specific block size or aspecific block depth. In addition, information about a specific blocksize or a specific block depth may be determined based on a codingparameter. In addition, information about a specific block size or aspecific block depth may be determined based on another encoded/decodedsyntax element value. A block in a sub-CTU may have a square ornon-square shape.

Here, in a block including a specific block size or having a largerblock size than a specific block size, at least one or more pieces ofinformation about motion compensation may be entropy encoded/decoded. Ina block having a smaller block size than a specific block size, at leastone or more pieces of information about motion compensation may not beentropy encoded/decoded

In a block having a smaller block size within a specific block size,motion compensation may be performed based on at least one or morepieces of entropy-encoded/decoded information about motion compensationin the specific block size. Here, in blocks having a smaller block sizewithin a specific block size, at least one or more among a motion vectorcandidate, a motion vector candidate list, a merge candidate, a mergecandidate list, or other information included in information aboutmotion compensation may be shared.

Here, in a block including a specific block depth or having a shallowerblock depth than a specific block depth, at least one or more pieces ofinformation about motion compensation may be entropy encoded/decoded. Ina block having a deeper block depth than a specific block depth, atleast one or more pieces of information about motion compensation maynot be entropy encoded/decoded.

In a block having a deeper block depth below a specific block depth,motion compensation may be performed based on at least one or morepieces of entropy-decoded information about motion compensation at thespecific block depth. In a block having a deeper block depth below aspecific block depth, motion compensation may be performed based on atleast one or more pieces of information about motion compensation at thespecific block depth, and information about motion compensation at thespecific block depth may be entropy encoded. Here, in blocks having adeeper block depth below a specific block depth, at least one or moreamong a motion vector candidate, a motion vector candidate list, a mergecandidate, a merge candidate list, or other information included ininformation about motion compensation may be shared.

For example, when the block size of a CTU is 64×64 and at least one ormore pieces of information about motion compensation in a 32×32 block,which is a sub-CTU, are entropy encoded/decoded, motion compensation ina block belonging to the 32×32 block but having a smaller size may beperformed based on at least one or more pieces ofentropy-encoded/decoded information about motion compensation in the32×32 block unit.

For example, when the block size of a CTU is 128×128 and at least one ormore pieces of information about motion compensation in a 16×16 block,which is a sub-CTU, are entropy encoded/decoded, motion compensation ina block belonging to the 16×16 block but having a smaller or equal sizeto the 16×16 block unit may be performed based on at least one or morepieces of entropy-encoded/decoded information about motion compensationin the 16×16 block unit.

For example, when the block depth of a CTU is 0 and at least one or morepieces of information about motion compensation at a block depth of 1,that is in a sub-CTU, are entropy encoded/decoded, motion compensationin a block belonging to the block depth of 1 but having a deeper blockdepth may be performed based on at least one or more pieces ofentropy-encoded/decoded information about motion compensation at theblock depth of 1.

For example, when the block depth of a CTU is 0 and at least one or morepieces of information about motion compensation at a block depth of 2,that is in a sub-CTU, are entropy encoded/decoded, motion compensationin a block belonging to the block depth of 2 but having a deeper orequal block depth may be performed based on at least one or more piecesof entropy-encoded/decoded information about motion compensation at theblock depth of 2.

Here, as the positive integer for depth increases, the depth mayincrease. As the depth value decreases, the depth may become shallower.In addition, as a depth increases, a block size may decrease. On theother hand, as a depth decreases, a block size may increase. Inaddition, a sub-depth of a specific block depth may mean a deeper depththan the specific block depth. A sub-depth of a specific block depth maymean a deeper depth within a block size corresponding to the specificblock depth.

In addition, at least one or more pieces of the information about motioncompensation may be calculated by using index information within setsthat are determined beforehand in an encoder and a decoder.

Also, in at least one or more among a video parameter set, a sequenceparameter set, a picture parameter set, an adaptation parameter set, apicture header and a slice header, at least one or more pieces ofinformation about motion compensation may be entropy encoded/decoded.

Also, a difference value for at least one or more pieces of informationabout motion compensation in at least one or more units among CTU,sub-CTU, CU and PU may be entropy encoded/decoded by using at least oneor more pieces of information about motion compensation at higherlevels, which are the parameter sets, the picture header and the sliceheader, as prediction values. A value for at least one or more pieces ofinformation about motion compensation may be calculated by adding aprediction value for at least one or more pieces of information aboutmotion compensation and a difference value for at least one or morepieces of information about motion compensation.

Alternatively, at least one or more pieces of information about motioncompensation in at least one or more among CTU, sub-CTU, CU and PU maybe entropy encoded/decoded by using at least one or more pieces ofinformation about motion compensation in a specific region within apicture, a slice, a tile or a CTU as a prediction value. A value for atleast one or more pieces of information about motion compensation may becalculated by adding a prediction value for at least one or more piecesof information about motion compensation and a difference value for atleast one or more pieces of information about motion compensation.

In addition, at least one or more pieces of information about motioncompensation for a prediction block may be entropy encoded/decoded in atleast one or more specific regions within a picture, a slice, a tile ora CTU.

In addition, an encoder/decoder may entropy encode/decode a differencevalue for at least one or more pieces of information about motioncompensation by using at least one or more pieces of information aboutmotion compensation as a prediction value for at least one or morepieces of information about motion compensation of an encoded/decodedneighbor block. An encoder/decoder may calculate a value for at leastone or more pieces of information about motion compensation by adding aprediction value for at least one or more pieces of information aboutmotion compensation and a difference value for at least one or morepieces of information about motion compensation.

In addition, an encoder/decoder may use at least one or more pieces ofinformation about motion compensation of an encoded/decoded neighborblock as a value for at least one or more pieces of information aboutmotion compensation for a current block without performing entropyencoding/decoding for at least one or more pieces of information aboutmotion compensation.

In addition, an encoder/decoder may derive the at least one or morepieces of information about motion compensation on the basis of at leastone or more among coding parameters.

In addition, an encoder/decoder may entropy decode the at least one ormore pieces of information about motion compensation from a bitstream onthe basis of at least one or more among coding parameters. Anencoder/decoder may entropy encode the at least one or more pieces ofinformation about motion compensation into a bitstream on the basis ofat least one or more among coding parameters.

In addition, information about motion compensation may further includeat least one or more among a motion vector, motion vector resolutioninformation, overlapped block motion compensation information, localillumination compensation information, affine motion compensationinformation, decoder-side motion vector derivation information, andbi-directional optical flow information. Here, the decoder-side motionvector derivation may mean pattern matched motion vector derivation.

Motion vector resolution information may be information about whether ornot a specific resolution is used for at least one or more of a motionvector and a motion vector difference. Here, a resolution may mean aprecision. In addition, a specific resolution may be at least one ormore among 16-pixel (16-pel) unit, 8-pixel (8-pel) unit, 4-pixel (4-pel)unit, integer-pixel (integer-pel) unit, ½-pixel (½-pel) unit, ¼-pixel(¼-pel) unit, ⅛-pixel (⅛-pel) unit, 1/16-pixel ( 1/16-pel) unit,1/32-pixel ( 1/32-pel) unit, and 1/64-pixel ( 1/64-pel) unit.

Overlapped block motion compensation information may be informationabout whether or not a sub-block, which is constructed by using motioninformation of a spatially neighbor block of an encoding/decoding targetblock while motion compensation of the encoding/decoding target block,is used to construct a prediction block of the encoding/decoding targetblock.

Local illumination compensation information may be information aboutwhether or not at least one or more of a weighting factor and an offsetvalue are applied when a prediction block of an encoding/decoding targetblock is constructed. Here, a weighting factor and an offset value maybe values that are calculated based on a reference block.

Affine motion compensation information may be information about whetheror not an affine motion model is used for motion compensation for anencoding/decoding target block. Here, an affine motion model may be amethod of partitioning one block into a multiplicity of sub-blocksthrough a multiplicity of parameters and calculating a motion vector ofa partitioned sub-block from representative motion vectors.

Decoder-side motion vector derivation information may be informationabout whether or not a motion vector necessary for motion compensationis derived from a decoder and is used. Here, information about a motionvector may not be entropy encoded/decoded. Here, when a merge mode isused, decoder-side motion vector derivation may be performed.

Bi-directional optical flow information may be information about whetheror not motion compensation is performed by correcting a motion vector inpixels. Here, a motion vector in pixels may not be entropyencoded/decoded. In addition, motion vector correction may change amotion vector value in blocks into pixels.

A current block may perform motion compensation by using at least one ormore pieces of entropy-decoded information about motion compensation. Acurrent block may perform motion compensation by using at least one ormore pieces of information about motion compensation and may entropyencode at least one or more pieces of information about motioncompensation.

When entropy encoding/decoding information about motion compensation ofa current encoding/decoding target block, an encoder/decoder may notperform entropy encoding/decoding for at least one or more pieces ofinformation about the motion compensation by using encoding informationof a current picture, to which the current encoding/decoding targetblock belongs, and reference pictures in a reference picture list.

Here, encoding information of a current picture and reference picturesmay be POC (Picture Order Count) information. An encoder/decoder may notperform entropy encoding/decoding for at least one or more pieces ofinformation about the motion compensation by using POC information of acurrent picture and reference pictures in a reference picture list.

When POC information among a current picture and reference pictures in areference picture list satisfies the following conditions, anencoder/decoder may entropy encode/decode information (for example,sym_mvd_flag) indicating whether or not entropy encoding/decoding isperformed for L0 and L1 reference picture index information (ref_idx_l0and ref_idx_l1) and uni-directional (L0 or L1) motion vector differenceinformation (MvdL0 or MvdL1) for a current encoding/decoding targetblock.

[Condition 1] A current encoding/decoding target block is abi-prediction mode and there are L0 reference picture and L1 referencepicture.

[Condition 2-1] At least one or more reference picture POCs among Nreference pictures in L0 reference picture list are smaller than acurrent picture POC, and at least one or more reference picture POCsamong M reference pictures in L1 reference picture list are larger thana current picture POC (N and M may be natural numbers greater than 0 andmay be equal to or different from each other).

[Condition 2-2] At least one or more reference picture POCs among Nreference pictures in L0 reference picture list are larger than acurrent picture POC, and at least one or more reference picture POCsamong M reference pictures in L1 reference picture list are smaller thana current picture POC (N and M may be natural numbers greater than 0 andmay be equal to or different from each other).

The [Condition 2-2] may be performed when there is no reference picturesatisfying the [Condition 2-1]. On the contrary, when there is noreference picture satisfying [Condition 2-2] after [Condition 2-2] isperformed, [Condition 2-1] may be performed.

In case the conditions are satisfied, L0 and L1 reference picture indexinformation and uni-directional (L0 or L1) motion vector difference(MVD) information may be always derived without entropyencoding/decoding, irrespective of information indicating whether or notentropy encoding/decoding is performed (for example, ‘sym_mvd_flag’).

When the conditions are satisfied and an encoded/decoded ‘sym_mvd_flag’has a first value (0), entropy encoding/decoding for L0 and L1 referencepicture index information and MVD (Motion Vector Difference) informationof L0 and L1 may be performed.

When the conditions are satisfied and an encoded/decoded ‘sym_mvd_flag’has a second value (1), L0 and L1 reference picture index informationand MVD information of one direction (L0 or L1) may not be entropyencoded/decoded but be derived as follows.

When the [Condition 2-1] is satisfied, if POCs of at least one or morereference pictures among N reference pictures of L0 reference picturelist are smaller than a current picture POC, position information of areference picture being a short-term reference picture and having a POC,which is smaller than and has the smallest difference from the currentpicture POC, within L0 reference picture list may be derived as a L0reference picture index value.

For example, when a second reference picture POC of L0 reference picturelist has the smallest difference from a current picture POC, the value‘1’ indicating a second reference picture in L0 reference picture listmay be derived as L0 reference picture index.

When the [Condition 2-1] is satisfied, if POCs of at least one or morereference pictures among M reference pictures of L1 reference picturelist are larger than a current picture POC, position information of areference picture being a short-term reference picture and having a POC,which is larger than and has the smallest difference from the currentpicture POC, within L1 reference picture list may be derived as a Lreference picture index value.

For example, when a third reference picture POC of L1 reference picturelist has the smallest difference from a current picture POC, the value‘2’ indicating a third reference picture in L1 reference picture listmay be derived as L i reference picture index.

When the [Condition 2-2] is satisfied, if POCs of at least one or morereference pictures among N reference pictures of L0 reference picturelist are larger than a current picture POC, position information of areference picture being a short-term reference picture and having a POC,which is larger than and has the smallest difference from the currentpicture POC, within L0 reference picture list may be derived as a L0reference picture index value.

For example, when a second reference picture POC of L0 reference picturelist has the smallest difference from a current picture POC, the value‘1’ indicating a second reference picture in L0 reference picture listmay be derived as L0 reference picture index.

When the [Condition 2-2] is satisfied, if POCs of at least one or morereference pictures among M reference pictures of L1 reference picturelist are smaller than a current picture POC, position information of areference picture being a current short-term reference picture andhaving a POC, which is smaller than and has the smallest difference fromthe current picture POC, within L1 reference picture list may be derivedas a L1 reference picture index value.

For example, when a third reference picture POC of L1 reference picturelist has the smallest difference from a current picture POC, the value‘2’ indicating a third reference picture in L1 reference picture listmay be derived as L1 reference picture index.

The check for the conditions [2-1] and [2-2] and the derivation processof L0/L1 reference picture indexes may be performed after a referencepicture list for a current picture or slice including anencoding/decoding target block is configured.

Without considering a POC difference among a current picture POC, a L0reference picture POC and a L1 reference picture POC, which areindicated by the derived L0 reference picture index and L1 referencepicture index information, an encoder/decoder may always derive a motionvector difference value of a direction that is not entropyencoded/decoded as follows.

When a motion vector difference value in the direction L0 (MVD0) isentropy encoded/decoded, a motion vector difference value in thedirection L1 (MVD1) may be derived as −MVD0. In other words, thehorizontal and vertical motion vector difference values in the directionL1 are as follows. (MVD1_x=−MVD0_x, MVD1_y=−MVD0_y)

When a motion vector difference value in the direction L1 (MVD1) isentropy encoded/decoded, a motion vector difference value in thedirection L0 (MVD0) may be derived as −MVD1. In other words, thehorizontal and vertical motion vector difference values in the directionL0 are as follows. (MVD0_x=−MVD1_x, MVD0_y=−MVD1_y)

An encoder/decoder may derive a motion vector difference value of adirection, which is not encoded/decoded, by considering a currentpicture POC, a L0 reference picture POC indicated by the derived L0reference picture index information, and a L1 reference picture POCindicated by the derived L1 reference picture index information.

For example, when a motion vector difference value in the direction L0(MVD0) is entropy encoded/decoded, a motion vector difference value inthe direction L1 may be derived as a scaled MVD0 value through Equation1 below.

                                      Equation  1curr? = DiffPicOrderCnt(currPic, RefPicList 0[refIdxL 0])currPocDiff? = DiffPicOrderCnt(currPic, RefPicList 1(refdxL 1])td = Clip 3(−128, 127, currPocDiffL 0)? = Clip?(−128, 127, currPocDiffL 1)   [???1] tx = (16? + (?(td)??1))/tddistScaleFactor = Clip 3(−4096, 4095, (???32)???)MVD1_x = Clip 3(??? − 1, Sign(distScaleFactor?MVD0_x)?((Abs(distScaleFactor?MVD0_x) + 127)???)MVD1_y = Clip 3(?, ? − 1, Sign(distScaleFactor?MVD0_y)?((Abs(distScaleFactor  ? MVD0_y)?127)???))?indicates text missing or illegible when filed                     

currPic is a POC of a current picture, RefPicList0[refIdxL0] is areference picture POC indicated by the derived L0 reference pictureindex information, and RefPicList1[refIdxL1] is a reference picture POCindicated by the derived L1 reference picture index information.

DiffPicOrderCnt( ) is a POC difference value between a current picturePOC and a reference picture POC.

For example, when a motion vector difference value in the direction L1(MVD01 is entropy encoded/decoded, a motion vector difference value inthe direction L0 may be derived as a scaled MVD1 value through Equation1.

An encoder/decoder may determine a direction of anentropy-encoded/decoded motion vector difference value by considering aPOC difference among a current picture POC, a L0 reference picture POCand a L1 reference picture POC, which are indicated by the derived L0reference picture index and L1 reference picture index information.

For example, when a difference between a current picture POC and thederived L0 reference picture POC is larger than a difference between acurrent picture POC and the derived L1 reference picture POC, a motionvector difference value in the direction L0 (MVD0) may be entropyencoded/decoded, and a motion vector difference value in the directionL1 may be derived as a scaled MVD0 value.

For example, when a difference between a current picture POC and thederived L1 reference picture POC is larger than a difference between acurrent picture POC and the derived L0 reference picture POC, a motionvector difference value in the direction L1 (MVD1) may be entropyencoded/decoded, and a motion vector difference value in the directionL0 may be derived as a scaled MVD1 value.

For another example, when POC information among a current picture andreference pictures in a reference picture list satisfies the followingconditions, an encoder/decoder may entropy encode/decode information(for example, sym_mvd_flag) indicating whether or not entropyencoding/decoding is performed for L0 and/or L1 reference picture indexinformation and uni-directional (L0 or L1) motion vector difference(MVD) information for a current encoding/decoding target block.

[Condition 1] A current encoding/decoding target block is abi-prediction mode and there are L0 reference picture and L1 referencepicture.

[Condition 2-1] At least one or more reference picture POCs among Nreference pictures in L0 reference picture list are smaller than acurrent picture POC, and at least one or more reference picture POCsamong M reference pictures in L1 reference picture list are larger thana current picture POC (N>0, M>0).

[Condition 2-2] At least one or more reference picture POCs among Nreference pictures in L0 reference picture list are larger than acurrent picture POC, and at least one or more reference picture POCsamong M reference pictures in L1 reference picture list are smaller thana current picture POC (N>0, M>0).

The [Condition 2-2] may be performed when there is no reference picturesatisfying the [Condition 2-1]. On the contrary, when there is noreference picture satisfying [Condition 2-2] after [Condition 2-2] isperformed, [Condition 2-1] may be performed.

[Condition 3] A POC difference between L0 reference picture and L1reference picture, which have the smallest POC difference from a currentpicture POC, satisfies Equation 2 below.

$\begin{matrix}{{{DiffPicOrderCnt}\left( {{currPic},{{RefPicList}\;{0\left\lbrack {{refIdx}\; 0} \right\rbrack}}} \right)}=={{DiffPicOrderCnt}\left( {{{RefPicList}\;{1\left\lbrack {{refIdx}\; 1} \right\rbrack}},{currPiC}} \right)}} & {{Equation}\mspace{14mu} 2}\end{matrix}$

currPic is a POC of a current picture, RefPicList0[refIdx0] is areference picture POC having the smallest POC difference from a currentpicture in L0 reference picture list, and RefPicList1[refIdx1] is areference picture POC having the smallest POC difference from a currentpicture in L1 reference picture list.

When the conditions are satisfied and an encoded/decoded ‘sym_mvd_flag’has a first value (0), an encoder/decoder may perform entropyencoding/decoding for L0 and L1 reference picture index information andMVD (Motion Vector Difference) information of L0 and L1.

When the conditions are satisfied and an encoded/decoded ‘sym_mvd_flag’has a second value (1), an encoder/decoder may not entropy encode/decodebut derive L0 and/or L1 reference picture index information and MVDinformation of one direction (L0 or L1), as follows.

In case the conditions are satisfied, an encoder/decoder may not entropyencode/decode but always derive L0 and/or L1 reference picture indexinformation and uni-directional (L0 or L1) motion vector difference(MVD) information, irrespective of information indicating whether or notentropy encoding/decoding is performed (for example, ‘sym_mvd_flag’).

A L0 reference picture index may be derived from the positioninformation of a reference picture having the smallest POC differencefrom a current picture in a L0 reference picture list.

For example, when a second reference picture POC of L0 reference picturelist has the smallest difference from a current picture POC, anencoder/decoder may derive the value ‘1’ indicating the second referencepicture in L0 reference picture list as L0 reference picture index.

A L1 reference picture index may be derived from the positioninformation of a reference picture having the smallest POC differencefrom a current picture in a L1 reference picture list.

For example, when a second reference picture POC of L0 reference picturelist has the smallest difference from a current picture POC, anencoder/decoder may derive the value ‘1’ indicating the second referencepicture in L0 reference picture list as L0 reference picture index.

The checking for the conditions [2-1], [2-2] and [3] and the derivationprocess of L0/L1 reference picture indexes may be performed after areference picture list for a current picture or slice including anencoding/decoding target block is configured.

When a motion vector difference value in the direction L0 (MVD0) isentropy encoded/decoded, a motion vector difference value in thedirection L1 (MVD1) may be derived as −MVD0. In other words, thehorizontal and vertical motion vector difference values in the directionL1 are as follows. (MVD1_x=−MVD0_x, MVD1_y=−MVD0_y)

When a motion vector difference value in the direction L1 (MVD1) isentropy encoded/decoded, a motion vector difference value in thedirection L0 (MVD0) may be derived as −MVD1. In other words, thehorizontal and vertical motion vector difference values in the directionL0 are as follows. (MVD0_x=−MVD1_x, MVD_y=−MVD1_y)

For another example, when POC information among a current picture andreference pictures in a reference picture list satisfies the followingconditions, an encoder/decoder may entropy encode/decode information(for example, sym_mvd_flag) indicating whether or not entropyencoding/decoding is performed for L0 and/or L1 reference picture indexinformation and uni-directional (L0 or L1) motion vector difference(MVD) information for a current encoding/decoding target block.

[Condition 1] A current encoding/decoding target block is abi-prediction mode and there are L0 reference picture and L1 referencepicture.

[Condition 2] A POC difference among a current picture POC, a N-threference picture POC of L0 reference picture list, and a M-th referencepicture POC of L1 reference picture list satisfies Equation 3 below.

$\begin{matrix}{{{DiffPicOrderCnt}\left( {{currPic},{{RefPicList}\;{0\left\lbrack {N\text{-}1} \right\rbrack}}} \right)}=={{DiffPicOrderCnt}\left( {{{RefPicList}\;{1\left\lbrack {M\text{-}1} \right\rbrack}},{currPiC}} \right)}} & {{Equation}\mspace{14mu} 3}\end{matrix}$

currPic is a POC of a current picture, RefPicList0[N−1] is a N-threference picture POC of L0 reference picture list, and RefPicList1[M−1]is a M-th reference picture POC of L1 reference picture list.

Here, N and M may be natural numbers greater than 0 and have the samevalue or different values.

For example, when N and M have a value of ‘1’, they may mean a firstreference picture POC in a L0/L1 reference picture list.

When the conditions are satisfied and an encoded/decoded ‘sym_mvd_flag’has a first value (0), an encoder/decoder may perform entropyencoding/decoding for L0 and L1 reference picture index information andMVD (Motion Vector Difference) information of L0 and L1.

When the conditions are satisfied and an encoded/decoded ‘sym_mvd_flag’has a second value (1), an encoder/decoder may not entropy encode/decodebut derive L0 and/or L1 reference picture index information and MVDinformation of one direction (L0 or L1), as follows.

An L0 reference picture index may be derived as a value of ‘N−1’,thereby indicating a N-th reference picture of L0 reference picturelist.

For example, in the case of N=1, a L0 reference picture index may bederived as ‘0’, thereby indicating a first reference picture of L0reference picture list.

An L1 reference picture index may be derived as a value of ‘M−1’,thereby indicating a M-th reference picture of L1 reference picturelist.

For example, in the case of M=1, a L1 reference picture index may bederived as ‘0’, thereby indicating a first reference picture of L1reference picture list.

The check for the condition [2] and the derivation process of L0/L1reference picture indexes may be performed after a reference picturelist for a current picture or slice including an encoding/decodingtarget block is configured.

When a motion vector difference value in the direction L0 (MVD0) isentropy encoded/decoded, a motion vector difference value in thedirection L1 (MVD1) may be derived as −MVD0. In other words, thehorizontal and vertical motion vector difference values in the directionL1 are as follows. (MVD1_x=−MVD0_x, MVD1_y=−MVD0_y)

When a motion vector difference value in the direction L1 (MVD1) isentropy encoded/decoded, a motion vector difference value in thedirection L0 (MVD0) may be derived as −MVD1. In other words, thehorizontal and vertical motion vector difference values in the directionL0 are as follows. (MVD0_x=−MVD1_x, MVD_y=−MVD_y)

In case the conditions are satisfied, L0 and/or L1 reference pictureindex information and uni-directional (L0 or L1) motion vectordifference (MVD) information may be always derived without entropyencoding/decoding, irrespective of information indicating whether or notentropy encoding/decoding is performed (for example, ‘sym_mvd_flag’).

When a current picture is a B slice and the POC information among acurrent picture POC and reference pictures in a reference picture listsatisfies at least one or more conditions described above, an encodermay entropy encode information (for example, smvd_enabled_flag)indicating whether or not information (for example, sym_mvd_flag)indicating the possibility of entropy decoding for ‘L0 and/or L1reference picture index information and uni-directional (L0 or L1)motion vector difference (MVD) information’ for all the encoding targetblocks in a current slice can be transmitted, and transmit theinformation to a decoder at levels like sequence parameter set (SPS),picture parameter set (PPS), adaptation parameter set (APS), pictureheader, tile group header, slice header and CTU.

A decoder may entropy decode the corresponding information (for example,smvd_enabled_flag), which is transmitted from an encoder, and may decode‘sym_mvd_flag’ information for a current decoding target block on thebasis of the corresponding information. Here, smvd_enabled_flagindicating that sym_mvd_flag can be transmitted may be signaled atlevels like sequence parameter set (SPS), picture parameter set (PPS),adaptation parameter set (APS), picture header, tile group header, sliceheader and CTU.

Accordingly, when the following conditions are satisfied, anencoder/decoder may entropy encode/decode information (for example,sym_mvd_flag) indicating whether or not entropy encoding/decoding isperformed for L0 and L1 reference picture index information (ref_idx_l0and ref_idx_l1) and uni-directional (L0 or L1) motion vector differenceinformation (MvdL0 or MvdL1) for a current encoding/decoding targetblock.

[Condition 1] A current encoding/decoding target block is abi-prediction mode and there are L0 reference picture and L1 referencepicture.

[Condition 2-1] At least one or more reference picture POCs among Nreference pictures in L0 reference picture list are smaller than acurrent picture POC, and at least one or more reference picture POCsamong M reference pictures in L1 reference picture list are larger thana current picture POC (N and M may be natural numbers greater than 0 andmay be equal to or different from each other).

[Condition 2-2] At least one or more reference picture POCs among Nreference pictures in L0 reference picture list are larger than acurrent picture POC, and at least one or more reference picture POCsamong M reference pictures in L1 reference picture list are smaller thana current picture POC (N and M may be natural numbers greater than 0 andmay be equal to or different from each other).

[Condition 3] smvd_enabled_flag has a value indicating that sym_mvd_flagcan be transmitted (for example, smvd_enabled_flag is ‘1’)

The [Condition 2-2] may be performed when there is no reference picturesatisfying the [Condition 2-1]. On the contrary, when there is noreference picture satisfying [Condition 2-2] after [Condition 2-2] isperformed, [Condition 2-1] may be performed.

For example, an encoder/decoder may decode ‘sym_mvd_flag’ informationonly when ‘tile_proup_smvd_enabled_flag’ transmitted from a tile groupheader has a second value (1) and a decoding target block is abi-prediction mode. When ‘sym_mvd_flag’ has a second value (1), theencoder/decoder may derive L0 and/or L1 reference picture indexinformation and uni-directional motion vector difference (MVD)information for the corresponding block by using at least one or moreamong the above-described methods.

For example, when ‘tile_goup_smvd_enabled_flag’ transmitted from a tilegroup header has a first value (0), an encoder/decoder may not decode‘sym_mvd_flag’ information for all the decoding target blocks but inferthe corresponding information as the first value (0) and thus performentropy decoding for ‘L0 and L1 reference picture index information andL0 motion vector difference/L1 motion vector difference’ information.

A first value and a second value are not limited to the above example.The present invention may include a case where the same definition as inthe above-described embodiment is applied while the first value is 1 andthe second value is 0.

When a current picture is a B slice and the POC information among acurrent picture POC and reference pictures in a reference picture listsatisfies at least one or more conditions described above, an encodermay entropy encode information (for example,‘tile_group_smvd_enabled_flag’) indicating whether or not information(for example, ‘sym_mvd_flag’) indicating the possibility of entropydecoding for ‘L0 and/or L1 reference picture index information anduni-directional (L0 or L1) motion vector difference (MVD) information’for all the encoding target blocks in a current slice can betransmitted, at at least one level among sequence parameter set (SPS),picture parameter set (PPS), adaptation parameter set (APS), pictureheader, tile group header, slice header, CTU and CU, and may transmitthe information to a decoder.

A decoder may entropy decode the corresponding information (for example,‘tile_group_smvd_enabled_flag’), which is transmitted from an encoder,and may decode ‘sym_mvd_flag’ information for a current decoding targetblock on the basis of the corresponding information.

In addition, when at least one or more among the above-described POCconditions are satisfied, an encoder may entropy encode positioninformation of L0 and/or L1 reference pictures at at least one levelamong picture header, tile group header, slice header and CTU andtransmit the information to a decoder.

For example, as shown in FIG. 13, when ‘tile_group_smvd_enabled_flag’that is entropy decoded in a tile group header has a second value (‘1’),a decoder may entropy decode reference picture index information (forexample, smvd_ref_idx0, smvd_ref_idx1) commonly applicable to all thetarget blocks included in a tile group. When ‘sym_mvd_flag’ informationthat is entropy decoded in a decoding target block has a second value(‘1’), the decoder may derive a reference picture for the correspondingblock from reference picture index information (for example,smvd_ref_idx0 or smvd_ref_idx1) that is entropy decoded in a tile groupheader. The tile group in FIG. 13 may mean a slice.

A first value and a second value are not limited to the above example.The present invention may include a case where the same definition as inthe above-described embodiment is applied while the first value is 1 andthe second value is 0.

When a current picture is a B slice and the POC information among acurrent picture POC and reference pictures in a reference picture listsatisfies at least one or more conditions described above, an encodermay entropy encode information (for example,‘tile_group_smvd_enabled_flag’) indicating whether or not information(for example, ‘sym_mvd_flag’) indicating the possibility of entropydecoding for ‘L0 and/or L1 reference picture index information anduni-directional (L0 or L1) motion vector difference (MVD) information’for all the encoding target blocks in a current tile group can betransmitted, at at least one level among sequence parameter set (SPS),picture parameter set (PPS), adaptation parameter set (APS), pictureheader, tile group header, slice header, CTU and CU, and may transmitthe information to a decoder.

A decoder may decode ‘sym_mvd_flag’ information for a current decodingtarget block on the basis of the corresponding information (for example,‘tile_group_smvd_enabled_flag’) transmitted from an encoder.

In addition, when at least one or more among the above-described POCconditions are satisfied, an encoder may entropy encode positioninformation of L0 and/or L1 reference pictures at at least one levelamong tile group header, slice header and CTU and transmit theinformation to a decoder.

For example, as shown in FIG. 14, when ‘tile_group_smvd_enabled_flag’that is entropy decoded in a tile group header has a second value (‘1’),a decoder may entropy decode information (for example,‘default_smvd_refIdx_flag’) indicating whether or not default referencepicture index information is used which is commonly applicable to allthe target blocks included in a tile group. In addition, when‘default_smvd_refIdx_flag’ has a second value (‘1’), a decoder may applya reference picture index, which indicates a L0/L reference picture in apredetermined position defined by an encoder/decoder, to all the targetblocks. The tile group in FIG. 14 may mean a slice.

For example, a reference picture index indicating a predeterminedposition that is defined in an encoder/decoder may be ‘0’ indicating afirst reference picture of an L0 reference picture list and an L1reference picture list. When the corresponding flag(‘default_smvd_refIdx_flag’) has a first value (0), reference pictureindex information (for example, smvd_ref_idx0 and/or smvd_ref_idx1)commonly applicable to all the target blocks in a tile group may beadditionally entropy decoded. When ‘sym_mvd_flag’ information that isentropy decoded in a decoding target block has a second value (‘1’), areference picture for the corresponding block may be derived from areference picture index (for example, smvd_ref_idx0 and/orsmvd_ref_idx1) that is entropy decoded in a tile group header.

sps_smvd_enable flag of FIG. 15 is information that is transmitted at asequence level. This may be information, indicating whether or not asymmetric MVD mode is used, which is derived without entropy decoding ofuni-directional motion vector difference information and bi-directionalreference picture index information of an encoding/decoding targetblock.

mvd_l1_zero_flag of FIG. 16 may be information indicating that a motionvector difference value of L1 direction is not decoded but derived as(0, 0). When mvd_l1_zero_flag has a first value (1), it may indicatethat a motion vector difference value of L1 direction is not decoded butderived as (0, 0). When mvd_l1_zero_flag has a second value (0), it mayindicate that a motion vector difference value of L1 direction is notderived as (0, 0).

Accordingly, when the following conditions are satisfied, anencoder/decoder may entropy encode/decode information (for example,sym_mvd_flag) indicating whether or not entropy encoding/decoding isperformed for L0 and L1 reference picture index information (ref_idx_l0and ref_idx_l1) and uni-directional (L0 or L1) motion vector differenceinformation (MvdL0 or MvdL1) for a current encoding/decoding targetblock.

[Condition 1] A current encoding/decoding target block is abi-prediction mode and there are L0 reference picture and L1 referencepicture.

[Condition 2-1] At least one or more reference picture POCs among Nreference pictures in L0 reference picture list are smaller than acurrent picture POC, and at least one or more reference picture POCsamong M reference pictures in L1 reference picture list are larger thana current picture POC (N and M may be natural numbers greater than 0 andmay be equal to or different from each other).

[Condition 2-2] At least one or more reference picture POCs among Nreference pictures in L0 reference picture list are larger than acurrent picture POC, and at least one or more reference picture POCsamong M reference pictures in L1 reference picture list are smaller thana current picture POC (N and M may be natural numbers greater than 0 andmay be equal to or different from each other).

[Condition 3] sps_smvd_enabled_flag has a value indicating thatsym_mvd_flag can be transmitted (for example, sps_smvd_enabled_flag is‘1’).

[Condition 4] mvd_l1_zero_flag has a value indicating that a motionvector difference value of L1 direction is not derived as (0, 0) (forexample, mvd_l1_zero_flag is ‘0’).

The [Condition 2-2] may be performed when there is no reference picturesatisfying the [Condition 2-1]. On the contrary, when there is noreference picture satisfying [Condition 2-2] after [Condition 2-2] isperformed, [Condition 2-1] may be performed.

Meanwhile, when the ‘mvd_l1_zero_flag’ information has a second value(1), at least one or more among ‘tile_group_smvd_enabled_flag’,‘default_smvd_refIdx_flag’, ‘smvd_ref_idx0’, ‘smvd_ref_idx1’, and‘sym_mvd_flag’, which are described above, may not be entropyencoded/decoded. In addition, although the ‘mvd_l1_zero_flag’ wasdescribed to be encoded/decoded in a tile group header, it may beencoded/decoded at at least one level among sequence parameter set(SPS), picture parameter set (PPS), adaptation parameter set (APS),picture header, slice header, CTU and CU.

FIG. 16 to FIG. 18 are other embodiments of symmetric MVD mode.

In FIG. 16, ‘sym_mvd_ref_idx[i]’ that is entropy decoded in a tile groupheader may be signaled only when each reference picture list has atleast one or more reference pictures.

For example, when there is one reference picture in L0 reference picturelist and there are two or more reference pictures in L1 referencepicture list, ‘sym_mvd_ref_idx[1]’ may be entropy decoded only for L1direction, and ‘sym_mvd_ref_idx[0]’ may be inferred as 0 for L0direction.

For another example, only one ‘sym_mvd_ref_idx’ that is applied to bothL0/L1 reference picture lists may be entropy decoded.

For example, when ‘tile_group_smvd_enabled_flag’ has a second value (1),a decoder may entropy decode only one ‘sym_mvd_ref_idx’ and use L0 andL1 reference pictures indicated by a corresponding value.

When a decoder needs to generate a bitstream satisfying at least one ormore among [Condition A], [Condition B] and [Condition C], which aredescribed in FIG. 17, the decoder should be able to decode a bitstreamsatisfying at least one or more among the conditions.

[Condition A]

A POC difference between a current picture and a reference pictureindicated by ‘symn_mvd_ref_idx[0]’ and a POC difference between acurrent picture and a reference picture indicated by‘sym_mvd_ref_idx[1]’ should have different encoding, a POC differencebetween a current picture and a reference picture indicated by‘sym_mvd_ref_idx[0]’ in L0 reference picture list should be smallest,and a POC difference between a current picture and a reference pictureindicated by ‘sym_mvd_ref_idx[1]’ in L1 reference picture list should besmallest.

[Condition B]

A POC difference between a current picture and a reference pictureindicated by ‘sym_mvd_ref_idx[0]’ and a POC difference between a currentpicture and a reference picture indicated by ‘sym_mvd_ref_idx[1]’ shouldhave different encoding.

[Condition C]

A POC difference between a current picture and a reference pictureindicated by ‘sym_mvd_ref_idx[0]’ and a POC difference between a currentpicture and a reference picture indicated by ‘sym_mvd_ref_idx[1]’ shouldbe equal to each other, and a reference picture indicated by‘sym_mvd_ref_idx[0]’ and a reference picture indicated by‘sym_mvd_ref_idx[1]’ should be located in opposite directions from thecurrent picture.

Meanwhile, the tile group in FIG. 16 and FIG. 17 may mean a slice.

FIG. 19 is a view for explaining an image decoding method according toan embodiment of the present invention.

A decoder may obtain symmetric motion vector difference modeavailability information from a bitstream (S1901).

Here, the symmetric motion vector difference mode availabilityinformation may be sps_smvd_enable flag that is obtained at a sequencelevel. Detailed description of sps_smvd_enable flag is omitted heresince it is already described above.

In addition, a decoder may obtain first prediction direction's zeromotion vector difference information from a bitstream (S1902).Specifically, the first prediction direction's zero motion vectordifference information may indicate that the motion vector differencevalue of the first prediction direction is not decoded but derived as(0, 0).

Here, the first prediction direction's zero motion vector differenceinformation may be invd_l1_zero_flag obtained at a picture level.Detailed description of mvd_l1_zero_flag is omitted here since it isalready described above.

In addition, a decoder may obtain symmetric motion vector differencemode information of a current block from the bitstream on the basis ofthe symmetric motion vector difference mode availability information andthe first prediction direction's zero motion vector differenceinformation (S1903).

Here, the motion vector difference mode information may be theabove-described sym_mvd_flag.

In addition, a decoder may obtain first prediction direction's referencepicture index information, second prediction direction's referencepicture index information and a first prediction direction's motionvector difference value on the basis of the symmetric motion vectordifference mode information (S1904). Specifically, when the symmetricmotion vector difference mode information of the current block indicatesa symmetric motion vector difference mode, a decoder may obtain thefirst prediction direction's reference picture index information, thesecond prediction direction's reference picture index information andthe first prediction direction's motion vector difference value byderiving rather decoding from the bitstream.

When the symmetric motion vector difference mode information of thecurrent block indicates a symmetric motion vector difference mode, thefirst prediction direction's motion vector difference value may bederived based on a second prediction direction's motion vectordifference value of the current block.

When the symmetric motion vector difference mode information of thecurrent block indicates a symmetric motion vector difference mode, thefirst prediction direction's reference picture index information may bederived as an index of a backward reference picture that is nearest to acurrent picture in a first prediction direction's reference picturelist, and the second prediction direction's reference picture indexinformation may be derived as an index of a forward reference picturethat is nearest to the current picture in a second predictiondirection's reference picture list.

When the symmetric motion vector difference mode information of thecurrent block indicates a symmetric motion vector difference mode, thefirst prediction direction's reference picture index information may bederived as an index of a forward reference picture that is nearest to acurrent picture in a first prediction direction's reference picturelist, and the second prediction direction's reference picture indexinformation may be derived as an index of a backward reference picturethat is nearest to the current picture in a second predictiondirection's reference picture list.

Meanwhile, when the symmetric motion vector difference mode informationof the current block indicates a symmetric motion vector differencemode, the first prediction direction's reference picture indexinformation and the second prediction direction's reference pictureindex information may be derived as an index of a short-term referencepicture.

In addition, a decoder may generate a prediction block of the currentblock by using at least one of the first prediction direction'sreference picture index information, the second prediction direction'sreference picture index information, and the first predictiondirection's motion vector difference value (S1905).

Here, the first prediction direction may be L1 prediction direction, andthe second prediction direction's may be L0 prediction direction.

FIG. 20 is a view for explaining an image encoding method according toan embodiment of the present invention.

An encoder may determine symmetric motion vector difference modeavailability information (S2001).

Here, the symmetric motion vector difference mode availabilityinformation may be sps_smvd_enable flag that is encoded at a sequencelevel. Detailed description of sps_smvd_enable flag is omitted heresince it is already described above.

In addition, an encoder may determine first prediction direction's zeromotion vector difference information (S2002). Specifically, the firstprediction direction's zero motion vector difference information mayindicate that the motion vector difference value of the first predictiondirection is not encoded but derived as (0, 0).

Here, the first prediction direction's zero motion vector differenceinformation may be mvd_l1_zero_flag encoded at a picture level. Detaileddescription of mvd_l1_zero_flag is omitted here since it is alreadydescribed above.

In addition, an encoder may encode symmetric motion vector differencemode information of a current block on the basis of the symmetric motionvector difference mode availability information and the first predictiondirection's zero motion vector difference information (S2003). Here, themotion vector difference mode information may be the above-describedsym_mvd_flag.

In addition, an encoder may determine, based on the symmetric motionvector difference mode information, whether or not first predictiondirection's reference picture index information, second predictiondirection's reference picture index information and a first predictiondirection's motion vector difference value are encoded (S2004).Specifically, when the symmetric motion vector difference modeinformation of the current block indicates a symmetric motion vectordifference mode, an encoder may determine as not encode the firstprediction direction's reference picture index information, the secondprediction direction's reference picture index information and the firstprediction direction's motion vector difference value.

When the symmetric motion vector difference mode information of thecurrent block indicates a symmetric motion vector difference mode, thefirst prediction direction's motion vector difference value may bederived based on a second prediction direction's motion vectordifference value of the current block.

When the symmetric motion vector difference mode information of thecurrent block indicates a symmetric motion vector difference mode, thefirst prediction direction's reference picture index information may bederived as an index of a backward reference picture that is nearest to acurrent picture in a first prediction direction's reference picturelist, and the second prediction direction's reference picture indexinformation may be derived as an index of a forward reference picturethat is nearest to the current picture in a second predictiondirection's reference picture list.

When the symmetric motion vector difference mode information of thecurrent block indicates a symmetric motion vector difference mode, thefirst prediction direction's reference picture index information may bederived as an index of a forward reference picture that is nearest to acurrent picture in a first prediction direction's reference picturelist, and the second prediction direction's reference picture indexinformation may be derived as an index of a backward reference picturethat is nearest to the current picture in a second predictiondirection's reference picture list.

Meanwhile, when the symmetric motion vector difference mode informationof the current block indicates a symmetric motion vector differencemode, the first prediction direction's reference picture indexinformation and the second prediction direction's reference pictureindex information may be derived as an index of a short-term referencepicture.

Here, the first prediction direction may be L1 prediction direction, andthe second prediction direction may be L0 prediction direction.

A bitstream generated by an image encoding method that is described inFIG. 20 may be stored in a non-transitory computer readable recordingmedium.

When at least one or more pieces of information about the motioncompensation are entropy encoded/decoded, at least one or more among thefollowing binarization methods may be used.

Truncated rice binarization method

K-th order Exp_Golomb binanzation method

Limited K-th order Exp_Golomb binarization method

Fixed-length binarization method

Unary binarization method

Truncated unary binarization method

When entropy encoding/decoding at least one or more pieces ofinformation about the motion compensation, an encoder/decoder maydetermine a context model by using at least one or more pieces ofinformation about motion compensation of a neighbor block, at least oneor more pieces of information about motion information that ispreviously encoded/decoded, information about a current unit/blockdepth, or information about a current unit/block size.

An encoder/decoder may entropy encode/decode at least one or more piecesof information about the motion compensation by using at least one ormore pieces of information about motion compensation of a neighborblock, at least one or more pieces of information about motioninformation that is previously encoded/decoded, information about acurrent unit/block depth, or information about a current unit/block sizeas a prediction value for information about motion compensation of acurrent block.

As described in the embodiment of the present invention, a referencepicture set used for reference picture list construction and referencepicture list modification may use at least one or more reference picturelists among L0, L1, L2 and L3.

When a boundary strength in a deblocking filter is calculated accordingto the embodiment of the present invention, 1 to N motion vectors of anencoding/decoding target block may be used. Here, N represents apositive integer of 1 or more and may be 2, 3, 4, or the like.

The embodiments of the present invention may also be applied when amotion vector for motion vector prediction has at least one or moreamong 16-pixel (16-pel) unit, 8-pixel (8-pel) unit, 4-pixel (4-pel)unit, integer-pixel (integer-pel) unit, ½-pixel (½-pel) unit, ¼-pixel(¼-pel) unit, ⅛-pixel (⅛-pel) unit, 1/16-pixel ( 1/16-pel) unit,1/32-pixel ( 1/32-pel) unit, and 1/64-pixel ( 1/64-pel) unit. Inaddition, when motion vector prediction is performed, a motion vectormay be selectively used for each of the pixel units.

A slice type to which the embodiments of the present invention areapplied may be defined, and the embodiments of the present invention maybe applied according to the corresponding slice type.

For example, when a slice type is T (Tri-predictive)-slice, a predictionblock may be generated using at least three or more motion vectors, anda weighted sum of at least three or more prediction blocks may becalculated and used as a final prediction block of an encoding/decodingtarget block. For example, when a slice type is Q(Quad-predictive)-slice, a prediction block may be generated using atleast four or more motion vectors, and a weighted sum of at least fouror more prediction blocks may be calculated and used as a finalprediction block of an encoding/decoding target block.

The above embodiments of the present invention may be applied not onlyto inter prediction and motion compensation methods using motion vectorprediction but also to inter prediction and motion compensation methodsusing a skip mode and a merge mode.

The above embodiments may be performed in the same method in an encoderand a decoder.

At least one or a combination of the above embodiments may be used toencode/decode a video.

A sequence of applying to above embodiment may be different between anencoder and a decoder, or the sequence applying to above embodiment maybe the same in the encoder and the decoder.

The above embodiment may be performed on each luma signal and chromasignal, or the above embodiment may be identically performed on luma andchroma signals.

A block form to which the above embodiments of the present invention areapplied may have a square form or a non-square form.

The above embodiment of the present invention may be applied dependingon a size of at least one of a coding block, a prediction block, atransform block, a block, a current block, a coding unit, a predictionunit, a transform unit, a unit, and a current unit. Herein, the size maybe defined as a minimum size or maximum size or both so that the aboveembodiments are applied, or may be defined as a fixed size to which theabove embodiment is applied. In addition, in the above embodiments, afirst embodiment may be applied to a first size, and a second embodimentmay be applied to a second size. In other words, the above embodimentsmay be applied in combination depending on a size. In addition, theabove embodiments may be applied when a size is equal to or greater thata minimum size and equal to or smaller than a maximum size. In otherwords, the above embodiments may be applied when a block size isincluded within a certain range.

For example, the above embodiments may be applied when a size of currentblock is 8×8 or greater. For example, the above embodiments may beapplied when a size of current block is 4×4 only. For example, the aboveembodiments may be applied when a size of current block is 16×16 orsmaller. For example, the above embodiments may be applied when a sizeof current block is equal to or greater than 16×16 and equal to orsmaller than 64×64.

The above embodiments of the present invention may be applied dependingon a temporal layer. In order to identify a temporal layer to which theabove embodiments may be applied, a corresponding identifier may besignaled, and the above embodiments may be applied to a specifiedtemporal layer identified by the corresponding identifier. Herein, theidentifier may be defined as the lowest layer or the highest layer orboth to which the above embodiment may be applied, or may be defined toindicate a specific layer to which the embodiment is applied. Inaddition, a fixed temporal layer to which the embodiment is applied maybe defined.

For example, the above embodiments may be applied when a temporal layerof a current image is the lowest layer. For example, the aboveembodiments may be applied when a temporal layer identifier of a currentimage is 1. For example, the above embodiments may be applied when atemporal layer of a current image is the highest layer.

A slice type or a tile group type to which the above embodiments of thepresent invention are applied may be defined, and the above embodimentsmay be applied depending on the corresponding slice type or tile grouptype.

In the above-described embodiments, the methods are described based onthe flowcharts with a series of steps or units, but the presentinvention is not limited to the order of the steps, and rather, somesteps may be performed simultaneously or in different order with othersteps. In addition, it should be appreciated by one of ordinary skill inthe art that the steps in the flowcharts do not exclude each other andthat other steps may be added to the flowcharts or some of the steps maybe deleted from the flowcharts without influencing the scope of thepresent invention.

The embodiments include various aspects of examples. All possiblecombinations for various aspects may not be described, but those skilledin the art will be able to recognize different combinations.Accordingly, the present invention may include all replacements,modifications, and changes within the scope of the claims.

The embodiments of the present invention may be implemented in a form ofprogram instructions, which are executable by various computercomponents, and recorded in a computer-readable recording medium. Thecomputer-readable recording medium may include stand-alone or acombination of program instructions, data files, data structures, etc.The program instructions recorded in the computer-readable recordingmedium may be specially designed and constructed for the presentinvention, or well-known to a person of ordinary skilled in computersoftware technology field. Examples of the computer-readable recordingmedium include magnetic recording media such as hard disks, floppydisks, and magnetic tapes; optical data storage media such as CD-ROMs orDVD-ROMs; magneto-optimum media such as floptical disks; and hardwaredevices, such as read-only memory (ROM), random-access memory (RAM),flash memory, etc., which are particularly structured to store andimplement the program instruction. Examples of the program instructionsinclude not only a mechanical language code formatted by a compiler butalso a high level language code that may be implemented by a computerusing an interpreter. The hardware devices may be configured to beoperated by one or more software modules or vice versa to conduct theprocesses according to the present invention.

Although the present invention has been described in terms of specificitems such as detailed elements as well as the limited embodiments andthe drawings, they are only provided to help more general understandingof the invention, and the present invention is not limited to the aboveembodiments. It will be appreciated by those skilled in the art to whichthe present invention pertains that various modifications and changesmay be made from the above description.

Therefore, the spirit of the present invention shall not be limited tothe above-described embodiments, and the entire scope of the appendedclaims and their equivalents will fall within the scope and spirit ofthe invention.

INDUSTRIAL APPLICABILITY

The present invention may be used to encode or decode an image.

1. An image decoding method, the method comprising: obtaining symmetricmotion vector difference mode availability information from a bitstream;obtaining first prediction direction's zero motion vector differenceinformation from the bitstream; obtaining symmetric motion vectordifference mode information of a current block from the bitstream on thebasis of the symmetric motion vector difference mode availabilityinformation and the first prediction direction's zero motion vectordifference information; obtaining first prediction direction's referencepicture index information, second prediction direction's referencepicture index information and a first prediction direction's motionvector difference value on the basis of the symmetric motion vectordifference mode information; and generating a prediction block of thecurrent block by using at least one of the first prediction direction'sreference picture index information, the second prediction direction'sreference picture index information and the first prediction direction'smotion vector difference value, wherein the obtaining of the firstprediction direction's reference picture index information, the secondprediction direction's reference picture index information and the firstprediction direction's motion vector difference value obtains the firstprediction direction's reference picture index information, the secondprediction direction's reference picture index information and the firstprediction direction's motion vector difference value by deriving ratherthan decoding from the bitstream when the symmetric motion vectordifference mode information of the current block indicates a symmetricmotion vector difference mode.
 2. The method of claim 1, wherein thefirst prediction direction is L1 prediction direction, and the secondprediction direction is L0 prediction direction.
 3. The method of claim1, wherein the first prediction direction's zero motion vectordifference information indicates that the motion vector difference valueof the first prediction direction is not decoded but derived as (0, 0).4. The method of claim 1, wherein the first prediction direction's zeromotion vector difference information is obtained at a picture level. 5.The method of claim 1, wherein the symmetric motion vector differencemode availability information is obtained at a sequence level.
 6. Themethod of claim 1, wherein, when the symmetric motion vector differencemode information of the current block indicates a symmetric motionvector difference mode, the first prediction direction's motion vectordifference value is derived based on a second prediction direction'smotion vector difference value of the current block.
 7. The method ofclaim 1, wherein, when the symmetric motion vector difference modeinformation of the current block indicates a symmetric motion vectordifference mode, the first prediction direction's reference pictureindex information is derived as an index of a backward reference picturethat is nearest to a current picture in a first prediction direction'sreference picture list, and the second prediction direction's referencepicture index information is derived as an index of a forward referencepicture that is nearest to the current picture in a second predictiondirection's reference picture list.
 8. The method of claim 1, wherein,when the symmetric motion vector difference mode information of thecurrent block indicates a symmetric motion vector difference mode, thefirst prediction direction's reference picture index information isderived as an index of a forward reference picture that is nearest to acurrent picture in a first prediction direction's reference picturelist, and the second prediction direction's reference picture indexinformation is derived as an index of a backward reference picture thatis nearest to the current picture in a second prediction direction'sreference picture list.
 9. The method of claim 7, wherein, when thesymmetric motion vector difference mode information of the current blockindicates a symmetric motion vector difference mode, the firstprediction direction's reference picture index information and thesecond prediction direction's reference picture index information arederived as an index of a short-term reference picture.
 10. An imageencoding method, the method comprising: determining symmetric motionvector difference mode availability information; determining firstprediction direction's zero motion vector difference information;encoding symmetric motion vector difference mode information of acurrent block on the basis of the symmetric motion vector differencemode availability information and the first prediction direction's zeromotion vector difference information; and determining, on the basis ofthe symmetric motion vector difference mode information, whether or notfirst prediction direction's reference picture index information, secondprediction direction's reference picture index information and a firstprediction direction's motion vector difference value are encoded,wherein the determining of whether or not the first predictiondirection's reference picture index information, the second predictiondirection's reference picture index information and the first predictiondirection's motion vector difference value are encoded is determined asnot encode the first prediction direction's reference picture indexinformation, the second prediction direction's reference picture indexinformation and the first prediction direction's motion vectordifference value, when the symmetric motion vector difference modeinformation of the current block indicates a symmetric motion vectordifference mode.
 11. The method of claim 10, wherein the firstprediction direction is L I prediction direction, and the secondprediction direction is L0 prediction direction.
 12. The method of claim10, wherein the first prediction direction's zero motion vectordifference information indicates that the motion vector difference valueof the first prediction direction is not encoded but derived as (0, 0).13. The method of claim 10, wherein the first prediction direction'szero motion vector difference information is encoded at a picture level.14. The method of claim 10, wherein the symmetric motion vectordifference mode availability information is encoded at a sequence level.15. The method of claim 10, wherein, when the symmetric motion vectordifference mode information of the current block indicates a symmetricmotion vector difference mode, the first prediction direction's motionvector difference value is derived based on a second predictiondirection's motion vector difference value of the current block.
 16. Themethod of claim 10, wherein, when the symmetric motion vector differencemode information of the current block indicates a symmetric motionvector difference mode, the first prediction direction's referencepicture index information is derived as an index of a backward referencepicture that is nearest to a current picture in a first predictiondirection's reference picture list, and the second predictiondirection's reference picture index information is derived as an indexof a forward reference picture that is nearest to the current picture ina second prediction direction's reference picture list.
 17. The methodof claim 10, wherein, when the symmetric motion vector difference modeinformation of the current block indicates a symmetric motion vectordifference mode, the first prediction direction's reference pictureindex information is derived as an index of a forward reference picturethat is nearest to a current picture in a first prediction direction'sreference picture list, and the second prediction direction's referencepicture index information is derived as an index of a backward referencepicture that is nearest to the current picture in a second predictiondirection's reference picture list.
 18. The method of claim 16, wherein,when the symmetric motion vector difference mode information of thecurrent block indicates a symmetric motion vector difference mode, thefirst prediction direction's reference picture index information and thesecond prediction direction's reference picture index information arederived as an index of a short-term reference picture.
 19. Anon-transitory computer readable recording medium storing a bitstreamgenerated by an image encoding method, wherein the image encoding methodcomprises determining symmetric motion vector difference modeavailability information; determining first prediction direction's zeromotion vector difference information; encoding symmetric motion vectordifference mode information of a current block on the basis of thesymmetric motion vector difference mode availability information and thefirst prediction direction's zero motion vector difference information;and determining, on the basis of the symmetric motion vector differencemode information, whether or not first prediction direction's referencepicture index information, second prediction direction's referencepicture index information and a first prediction direction's motionvector difference value are encoded, wherein the determining of whetheror not the first prediction direction's reference picture indexinformation, the second prediction direction's reference picture indexinformation and the first prediction direction's motion vectordifference value are encoded is determined as not encode the firstprediction direction's reference picture index information, the secondprediction direction's reference picture index information and the firstprediction direction's motion vector difference value, when thesymmetric motion vector difference mode information of the current blockindicates a symmetric motion vector difference mode.